1895.]

[Slade.

projecting process. The dental series in this suborder resembles that of the Insectivora, the molars being cuspid. Adopting the classification of the highest authorities, and notably that of Dr. Dobson, the Insectivora may be divided into two suborders, the Dermoptera and the Insectivora Vera. Accepting the above classification, the Insectivora, so far as concerns the jugal arch, may be brought into three groups :

1. Those in which the arch is complete and well developed, comprising the Tupaidæ, Macroscelidæ, Rhynchocyonidæ, Galeopithecida.

2. Those in which the arch is complete but more or less feebly devel oped, comprising the Erinaceida, Talpidæ, Chrysochloridæ.

3. Those in which the arch is partially or wholly deficient, comprising the Centetidæ, Potamogalidæ, Solenodontidæ, Soricidæ.

The Tupaia may be taken as a typical form of the first group. The jugal arch is well developed, a postorbital process from the frontal meeting a corresponding one from the malar, thus forming a complete bony orbital ring. The malar has a large longitudinal oval vacuity, which, although unique in this case, when taken with similar vacuities in the palate of this genus, as also in some of the other Insectivora, points unmistakably to the Marsupialia.

The horizontal curvature of the arch is sufficient to counteract any inherent weakness due to the vertical curvature with its convexity down. wards. The temporal fossa is moderately extended, while the coronoid surface of the mandible presents a large backward projecting surface, rising high above the transversely produced condyle.

In the second group, where the arch, although complete, is for the most part weak, the cranium presents marked modifications. In Erinaceus and Gymnura the arch is formed mostly by the processes of the squamosal and maxilla which join, while the malar is very small and occupies in a splintlike form the outer and under sides of the centre of the arch. There are no traces of any postorbital processes. The temporal fossa is deep and extended, while additional surface is afforded for the temporal muscle by the prominence of the sagittal and occipital crests. The ascending ramus of the mandible, with its broad, concave, coronoid surface, and the development of the pterygoid fossæ, denote increased masticatory powers, in spite of the apparent weakness of the buttress.

In the Talpida, certainly in all of the truly fossorial of the family, the jugal arch is slender, and exhibits no distinct malar bone, no occipital or sagittal crests, and no postorbital processes.

The mandible is long, and the vertical portion presents a moderately extended coronoid surface with a small transverse condyle. The infraorbital foramen is of great size, being a very slender osseous arch which serves for the transmission of the large infraorbital branch of the trifacial, affording the necessary supply of sensory nerves to the muzzle.

In the Chrysochloride, which in the general shape of the skull present modifications different from all other Insectivora, the jugal arch is in some species so expanded vertically, that, as Dr. Dobson remarks, "their upper PROC. AMER. PHILOS. 80C. XXXIV. 147. H. PRINTED MAY 8, 1895.

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[March 15, margins rise above the level of the cranium, giving additional origin to the large temporal muscles." There is no postorbital process given off either by the frontal or zygomatic arch. As regards the mandible, the coronoid process is little elevated, and in some species is nearly level with the transversely extended condyle.

In the third group the arch is incomplete, and in one instance, at least, may be described as entirely absent. In the Centetidæ, the skull is long and narrow, and marked by largely developed occipital and sagittal crests which serve as attachments for muscles of temporal origin. The zygomatic processes of the maxilla and squamosal are very short and rudimentary, while the malar is entirely absent. The temporal fossæ are very large, and the skull retains nearly the same width at their anterior and posterior regions. There is not a trace of a postorbital process. The infraorbital foramen is circular and capacious. There are no pterygoid fossæ. The coronoid process of the mandible is largely developed, its inner surface being concave, and its outer surface flattened. The condyle is small and circular, while the glenoid surface is transversely concave.

The other families of this group, with the exception of the Soricidæ, agree with the Centetida in the modifications of the skull that have been described. In the Soricidæ, the cranium is broadest just behind the glenoid surfaces. There is no jugal arch and no trace of a postorbital process. Frequently there is present a strongly marked lamboidal ridge as well as a sagittal crest. There is no pterygoid fossa, but very large vacuities exist on each side of the basis cranii. The mandible resembles that of the Talpida, although the horizontal ramus is shorter, while the ascending one "presents a very large and singularly deep excavation upon its internal surface quite characteristic of the genus." The articular surface of the condyle looks backwards instead of upwards. The angle of the jaw is elongated and thin. The infraorbital is large and bounded posteriorly by an osseous bar.

The jugal arch in the Rodentia is always present, and is generally complete, although it exhibits many modifications in its composition. Three bones form the arch, which is straight or slightly curved horizontally, while it almost invariably presents a curvature downwards. The position of the jugal therein serves as a determining character in grouping the various families of the order.

The temporal fossa is often small, showing feeble energy in the action of the temporal muscle. On the contrary, the pterygoid plates and fossæ are often largely increased in relation to the enlarged development of the muscular insertions. In close connection with these conditions, the coronoid process of the mandible is small and even rudimentary, while the parts about the angle are largely expanded. The condyle is little elevated and presents, with few exceptions, an antero-posterior articulating surface. Postorbital processes of the frontals exist in a few of the families, but in no case is there a corresponding process from the arch. The orbit is never separated from the temporal fossa.

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In many of the rodents there is present a more or less extensive dilatation of the infraorbital foramen, through which passes, in addition to the nerve, that portion of the masseter muscle which has its insertion upon the maxilla. This extends around the back of the jugal process of the maxilla in a pulley-like manner to an insertion just below the socket of the mandibular præmolar, and thus coöperates with the temporal in moving the mandible in a vertical direction. This attachment of a head of the masseter is peculiar to the order, and explains the use of the vacuity in the maxilla which oftentimes is of vast relative proportions. All existing rodents fall into two groups, the Simplicidentata and the Duplicidentata. The first embraces the Sciuromorpha, Hystricomorpha, Myomorpha, and the second, the Lagomorpha.

In the Sciuromorpha, the jugal forms the greater part of the arch, extending forward to the lachrymal and posteriorly to the glenoid cavity, of which it forms the outer wall, and is not supported below by a continuation backwards of the process of the maxilla. In the more typical forms there is no enlargement of the infraorbital opening, while the postorbital processes of the frontals are characteristic of the family Sciuridæ. The external pterygoid plate is entirely wanting and there is no fossa.

The arch in the Myomorpha is for the most part slender, and the jugal, which does not extend far forward, is supported by the continuation below of the maxillary process. The zygomatic process of the squamosal is short. No postorbital process of frontal exists. The infraorbital opening varies. In the family Muridæ, especially in the typical forms, this opening is perpendicular, wide above and narrow below, while the lower root of the zygomatic process of the maxilla is flattened into a thin perpendicular plate. Very much the same condition exists in the Myoxide, while in the Dipodidæ, the foramen is as large as the orbit, is rounded, and has a

separate canal for the nerve. The malar ascends to the lachrymal in a flattened plate. In close connection with these conditions the coronoid process of the mandible is small and rudimentary, while the parts around the angle of the ramus are much developed.

In the Hystricomorpha, the arch is stout. The jugal is not supported by the continuation of the maxillary process, and generally does not advance far forward. The infraorbital vacuity is large and is either triangular or oval. The coronary process and the condyle are but slightly elevated above the dental series.

In the Chinchillidæ, the jugal extends forward to the lachrymal. In the Dasyproctidæ, Cælogenys is characterized by the extraordinary develop. ment of the jugal arch, which presents an enormous vertical curvature, two-thirds of the anterior portion of which, constituting the maxilla, is hollowed out into a cavity which communicates with the mouth. The nerve passes through a separate canal, adjacent to the infraorbital opening. The jugal arch in the suborder Duplicidentata is well developed.

In the family Leporidæ, there are large wing-like postorbital processes, while the jugal, but feebly supported by the maxillary process, continues posteriorly to aid in the formation of the outer side of the glenoid articular surface, passing beneath the process of the squamosal.

The Lagomyidæ have no postorbital processes, and the posterior angle of the jugal is carried backward nearly to the auditory meatus. The infraorbital opening in the Duplicidentata is of the usual size. The angle of the jaw is rounded, and the coronoid process much produced upwards.

The Ungulata may at the present time be divided into the Ungulata vera, including the two suborders, Perissodactyla and Artiodactyla, and the Ungulata polydactyla, which comprises the two suborders, Hyracoidea and Proboscidea.

In its morphology, the jugal arch of the Ungulata presents various modifications. With few exceptions, two bones alone compose it, the squamosal and jugal, which are connected by a suture, the general direction of which is horizontal. Both the horizontal and vertical curvatures of the arch present considerable variations, as does also its relation to the neighboring parts.

In the Perissodactyla, the family Equidæ exhibits an arch, which, although relatively slender, is quite exceptional in its arrangement. The large and lengthened process of the squamosal not only joins the greatly developed postorbital process of the frontal, but passing beyond, forms a portion of the inferior and posterior boundary of the orbit. The malar spreading largely upon the cheek, sends back a nearly horizontal process to join the under surface of the squamosal process above described, while the orbit is entirely surrounded by a conspicuous ring of bone, thereby clearly determining the bounds between it and the temporal fossa, which last is remarkably small. This fossa is bounded above and posteriorly by more or less well-developed crests and ridges. The pterygoids are slender and delicate, without the presence of any fossa. The glenoid

surface is much extended transversely, concave from side to side, and bounded posteriorly by a prominent postglenoid process. The angle of the jaw is much expanded. The condyle is well elevated above the molar series, while the coronoid process is long, narrow and slightly recurved.

In the Rhinoceride and Tapiride, the arch is strongly developed, and composed of the squamosal and jugal processes, which are joined at about its centre by an oblique suture from above downwards, backwards and upwards. In the Tapiridæ, the arch is long, owing to the advanced position of the orbit. There is a small postorbital process, largest in the tapir, but the orbital and temporal fossæ are continuous. The surface for the temporal muscle is extensive. The glenoid fossa presents a transverse convex surface to articulate with the corresponding one of the mandible, which is not much elevated above the dental series. The coronoid process is slender and recurved, while the angle is broad, compressed, somewhat rounded and incurved.

In the Artiodactyla, the arch is slender and is composed of the process from the jugal, which passes backwards beneath the corresponding forward projecting process of the squamosal, the juncture being by a suture nearly horizontal in direction, and longest in the Cervidæ. The jugal sends up a postorbital process to meet the corresponding descending one of the frontal, the suture which unites them being about midway. The bony orbit is thus complete, while the jugal is forked posteriorly. The temporal region is relatively small. The horizontal curvature of the arch is very slight. The glenoid surface is extensive and slightly convex with a well-developed postglenoid process. The pterygoids present a large surface and are situated nearer the middle line than is the case in the Perissodactyla. The condyle is broad and flat, and the coronoid process is long, compressed, and slightly recurved. The angle is rounded and much expanded.

The Tylopoda alone among the Ruminantia have large surfaces with crests and ridges for the increased development of the temporal muscle. The horizontal curvature of the arch is greater than in the true ruminants, consequently the temporal fossa is wider and deeper-all in correlation with the powerful canine teeth. The forked articulation between the molar and squamosal is also more strongly marked.

Among the non-Ruminantia, the family Suidæ exhibit an arch in which the process of the jugal underlying the squamosal extends back to the glenoid fossa-the two bones being connected by a suture, which is vertical anteriorly for the depth of half the bone, and then horizontal. The postorbital process does not meet the frontal, in fact all traces of this are lost in Sus scrofa, but in the peccary and Barbaroussa it is quite prominent. The arch is short owing to the position of the orbit, and both vertical and horizontal curvatures are considerable. The narrow, transverse condylar surface of the mandible, and the small coronoid process with its