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rounded surface, are but slightly raised above the level of the alveolar surface. The pterygoid surface is extensive and the fossa deep.
In the Hippopotamidæ, the arch is broad and strong. Its superior border presents a marked sigmoid curvature, and the convexity which is always posterior, is in this case much shorter in proportion. The temporal fossæ, as also the surfaces for the muscular insertions, are extensive. The pterygoid surface is not so large as in the Suidæ. The glenoid fossa is slightly concave, but not bounded externally by a continuation of the jugal. The condyles of the mandible are nearly on a level with the molars, and the coronoid process is small and recurved. The angle is greatly modified for muscular attachment.
In the Hyracoidea, the arch is composed of three bones, of which the jugal is the most important. Resting anteriorly upon the maxilla, the jugal sends backwards a process to form the external boundary of the glenoid fossa. It also sends upwards a postorbital process to meet a corresponding one from the parietal alone or from the parietal and frontal combined, thus completing the bony orbit. Both horizontal and vertical curvatures are slight. The surface for the temporal muscle is largely developed while the pterygoid fosse are well marked. The ascending ramus of the mandible is high, and the angle is rounded and projects very much behind the condyle, which last is wide transversely, and rounded on its external border. The coronoid process is small, slightly recurved, and not on a level with the condylar surface.
In the Proboscidea, the arch is straight, slender, and composed of three bones. The maxilla forms the interior portion, while the jugal supported upon the process of the maxilla, meets that of the squamosal in the middle of the arch, and is continued under this as far as the posterior root. This modification is unlike that of any other ungulate. There is a small postorbital process from the frontal. The temporal surface is exten
sive, and that of the pterygoid considerable. The ascending ramus of the mandible is high, and the condyle small and round. The coronoid process is compressed, and but little elevated above the molar series, while the angle is thickened and rounded posteriorly.
The jugal arch in the order of the Cetacea presents some singular modifications. In the Delphinoidea, the squamosal, frontal and jugal enter into its composition. The squamosal sends forward a large, bulky process which nearly meets the descending postorbital process of the frontal. The jugal is an irregular flat bone, covered by the maxilla, and sends back from its anterior and internal border a long and very slender process, curved slightly downwards, to articulate with the short obtuse process of the squamosal, thereby forming the lower boundary of the orbit. So far as the relations of the squamosal and frontal are concerned, the portion of the arch thus formed is a counterpart of that of the horse : although the union of the two bones is much more complete in the latter animal. The jugal in the horse is relatively a much larger bone, and sends back a well-developed process which underlies that of the squamosal, with which it is joined by a nearly horizontal suture, thus forming a strong suborbital bony wall. The delicate character of the suborbital process of the jugal, and its union with the squamosal in the Delphinoidea, render it difficult at first sight to determine its relation to the arch, and yet when compared with that of the horse, its homological character cannot be disputed.
In the Balenoidea, much the same conditions are presented, except that the suborbital process of the jugal is both stronger and more curved. The small capacity of the temporal region, as well as the limited extent of the arch in the Cetacea, are correlated with the modifications presented by the mandible, in which the condylar surface is small, and looks directly backwards. There is no ascending ramus, and the coronoid process is quite rudimentary-all of which conditions are in direct relation to the nature of the food, and absence of the masticatory movements.
In the Sirenia, the arch is greatly developed, being composed of the squamosal and the jugal. The former of these is much thickened and presents upon its external face a smooth, convex surface. In the Manatus, this process of the squamosal rests loosely upon the process of the malar, which, underlying it, extends back as far as the glenoid, having first formed a rim which is both suborbital and postorbital, besides sending a broad plate downwards and backwards, thereby greatly increasing the vertical breadth. The orbital fossa is separated almost completely from the temporal by a bony partition. The surface for the muscular attachments, both of the temporal and masseter, are extensive, while the pterygoid plates and groove are relatively enlarged. The vertical curvature of the arch is great, but the horizontal is inconsiderable. The ascending ramus of the mandible is broad, compressed, with rounded angle and sur mounted by an obliquely placed, small convex condyle, much raised above the molar series. The coronary surface is broad, directed forwards, and but slightly elevated above the condyle.
In the Dugong, the arch is much less massive. There is no postorbital process from the jugal, and consequently no separation of the orbital and temporal fossæ by a bony orbit. The coronoid process of mandible looks backward.
Although the horizontal curvature of the arch is very slight in both genera of the Sirenia, the temporal fossæ are deepened and extended, conditions due to the walls of the cranium being compressed in a lateral direc tion, which materially increases the extent of surface for muscular attach*ment and development.
In the order Edentata, the jugal arch also offers unusual modifications. In the Myrmecophagidæ, it is very incomplete, being composed of the proximal end of the jugal, articulating with the narrow projecting process of the maxilla, and a very rudimentary fragment of the squamosal. These separate portions, however, do not meet, in fact they are widely separated. There is no boundary between the orbital and temporal fossæ, the latter being comparatively shallow. The glenoid fossa is a slight cavity running antero-posteriorly, and well adapted to the pointed, backward projecting condyles of the mandible, whose long straight horizontal rami present neither coronoid process nor angle. In Cycloturus, the mandible is somewhat arched, and presents a well-marked angular process, as well as a coronoid surface slightly recurved.
In the Bradypodidæ, containing the two forms, Bradypus and Cholo pus, the arch is imperfect, consisting of the jugal, which is narrow at its articulation with the lachrymal and maxilla, but which widening out into a broad compressed plate, terminates posteriorly in two processes, the upper pointing backwards and upwards, while the lower looks downwards and backwards. The straight process of the squamosal, although fairly developed, fails to meet either of those of the jugal. There is a postorbital process of the frontal, which is best marked in Cholopus. The glenoid is shallow and narrow from side to side. The mandible, widest in Cholapus, develops a rounded convex condylar surface, well raised up from the dental series, while the coronoid surface is large and recurved. The rounded angular process projects backwards to a considerable extent. The symphysis in both forms is solidified, while in Cholapus it projects forwards into a spout-like process. The temporal surface for muscular attachment is large, as also are the pterygoid plates. In the Dasypodidæ, the arch is complete, and in its formation the jugal largely enters. This bone extends from the lachrymal and frontal to the process of the squamosal, the anterior third of which it underlies. There is no postorbital process of the frontal. The glenoid presents a broad, slightly convex, transverse surface. The pterygoids are small. The mandible has a high ascending ramus, the condyle is transverse and above the alveoli, while the coronoid surface is large and the angle broad and projecting.
In the Manidæ, the arch is incomplete, owing to the absence of the malar, which if present would occupy almost the exact centre of the arch,
the length of the squamosal process and that of the maxilla being nearly equal on either side. The temporal and orbital fossæ form one depression in the side of the skull. The rami of the mandible are slender and straight and without teeth, angle, or coronoid process. The condyle is not raised above the level of the remainder of the ramus.
In the Orycteropidæ, the arch is complete, and the horizontal curvature is very slight. The postorbital process is well-developed. The mandible rises high posteriorly, with a coronoid slightly recurved, and with an ascending pointed process on the angular edge below the condyle.
In the Marsupialia, the jugal arch is always complete, and composed of the jugal, resting on the maxilla and squamosal, the first extending from the lachrymal anteriorly to the glenoid fossa posteriorly, of which it forms the external wall. The process of the squamosal passes above the jugal, being united to it by an almost horizontal suture. The horizontal and vertical curvatures of the arch are considerable, and the space for both temporal and masseter muscular insertions is extensive. The various ridges and crests are large, especially in the Dasyuride and Didelphidæ. The postorbital of the frontal is present as a rule, although in most forms inconsiderably developed. The ascending ramus of the mandible is less elevated than in several of the orders of the Mammalia. The condyle is but little raised above the molar series. The masseteric fossa is extremely projected at its lower external border, and the mandible, with one exception, has an inverted border to the angle.
In the Monetremata, the Echidnidæ possess an arch in which the squamosal is compressed, and sends forward a slender straight process to join the corresponding slight shaft-like process of the jugal. The horizontal curvature is extremely small.
In the Ornithorynchidæ, the arch is made up of the malar resting upon a process of the maxilla, which, passing straight backwards, unites with the squamosal process that rises far back on the sides of the cranium. While the mandible of the Echidna has but the rudiments of the parts which usually enter into its formation, that of the Ornithorynchus is more fully developed in relation to the attachment of the horny teeth.
In studying the significance of the jugal arch according as this portion of the mammalian cranium has been presented to us in the preceding pages, while there are modifications in certain groups which are somewhat difficult of explanation, we shall find that the general laws which govern its morphology may be satisfactorily determined. These laws, concisely speaking, are, that the development of the arch, as shown by the number of the bones, by the degree and the number of the curvatures, by its rela tion to the orbit and articulation of the mandible, as well as to other neighboring parts, and by the amount of surface presented for muscular insertion, all depend upon the energy and character of the masticatory muscles. That these in turn depend upon, and are closely correlated with the habits and environment of the animal.
The above laws are very clearly exhibited in the Carnivora and in the PROC. AMER. PHILOS. Soc. XXXIV. 147. I. PRINTED MAY 9, 1895.
Ungulata. In the Perissodactyla, the sagittal crest, ridges and extensive parietal surface are correlated with increased insertions of the temporal, while the large, strong and complicated arch has equal reference to a powerful masseter. So in the Artiodactyla, especially in the Ruminantia, the diminished surface for the temporal, and the smaller, weaker arch, both denote lessened energy in the above muscles, while the enlarged pterygoid muscular insertions show that the required action has been pro vided in another direction. As Prof. Cope has shown, "Forms which move the lower jaw transversely have the temporal muscles inversely as the extent of the lateral excursions of the jaw. Hence these have a diminished size in such forms as the ruminants, and are widely separated."
The singular fact that the Tylopoda alone of the selenodont Artiodactyla possess the sagittal crest, is explained by Prof. Cope, by the presence of canine teeth, which are used as weapons of offense and defense, and which demand large development of the temporal muscles.
The energy of the action of these muscles has reference to the position of the dental series. In the primitive Mammalia, as Cope shows, a considerable portion of the molar series is below and posterior to the vertical line of the orbit, and this condition has been preserved in the Rodentia and Proboscidia, forms which have the proal mastication. But in those which have lateral movements of the jaw, the molar series has gradually moved forwards. The camel alone retains the primitive condition.
The bunodont Artiodactyla, as the Dicotylidae, have the molar series posterior to the orbit; those with lateral movement of the jaw, the Suidæ, have them more anterior.
In the relation of the arch to the orbit, it is obvious that the position of this last must exert its influence upon the strength of the arch. When the orbit is above or below the articulation a longer and consequently a weaker arch is demanded, than when it is on a level with it. The same may be said when the orbit occupies an anterior rather than a posterior position upon the cranium. A comparison of the crania of the Tapirida with those of the Suide will corroborate this fact.
Then again the union of the bones by suture imparts a degree of elasticity to the arch which must serve to disperse over a given space the effects of shocks and blows, which might under other circumstances prove injurious.
We have already noted the peculiar vertical curvature of the arch downwards in the Rodentia. This is a decided manifestation of weakness, and is compensated in some of the families by the unusual arrangement made in the distribution of the muscular insertions of the masseter through the infraorbital opening, by which increased energy is imparted to the powers of mastication, and whereby the action of the mandible is rendered equal to the demand upon its efforts.
Where this does not exist, it is evident that the strength of the arch is still sufficient for the antero-posterior movement of the articulation so peculiar to the Rodentia and so characteristic of the act of gnawing. The