. times blend with, the supra-orbital plates, and circumscribe vacuities at the sides of the parietal region of the cranium. But the exterior of the skull is variously and singularly modified in the different Plagiostomous genera, development proceeding from the advanced cartilaginous stage just described, to establish peculiar plagiostomous characters, and to adapt the individual to its special sphere of existence. The same general confluence of cartilage, which pervades the protecting walls of the brain-case, characterises the appended arches of the cranium. A single strong suspensory pedicle, articulated to the side of the skull beneath the posterior angular (mastoid) process, has the hyoidean, and partly the mandibular *, arches attached to its lower end, the former by a close joint, the latter by two ligaments. The maxillary arch, in Squatina, is suspended by a ligament from its ascending or palatal process, to the notch between the vomerine and the anterior supra-cranial cartilaginous plate. From this point the jaw is continued in one direction forwards and inwards, completing the arch by meeting its fellow, to which it has a close ligamentous junction; and in the opposite direction, backwards and outwards, as a coalesced diverging appendage to the outer side of the tympanic pedicle, where it forms the more immediate articulation for the lower jaw, or mandibular arch, like the hypo-tympanic continuation of the upper maxillary bone in the Batrachia. Each lateral half or ramus consists of a single cartilage, the two being united together at the symphysis by ligament. Two slender labial cartilages are developed on each side the maxillary, and one on each side the mandibular arch; which complete the sides of the mouth. These cartilages Cuvier regarded as rudiments, respectively, of the intermaxillary, maxillary, and dentary bones; the dentigerous maxillary arch being his palatine bones, and the mandibular arch the articular piece of the lower jaw; but both palatines and articulars co-exist with labial cartilages, like those of the Squatina, in a Brazilian Torpedo (Narcine), and at the same time with distinct pterygoid cartilages. (xxI. 1835, pl. v. fig. 3. & 4.) † Four or five short cartilaginous rays, in Squatina, diverge from the posterior margin of the tympanic pedicle, and support a membrane answering to the opercular flap in Osseous fishes; in their ultimate homology these rays are the skeleton of the diverging appendage or limb of the tympano-mandibular arch. * Throughout these Lectures the term “ mandible” is applied to the lower jaw, and the inverted cranial arch which that jaw completes is called "mandibular : the arch formed by the upper jaw is called "maxillary." + It may be questioned whether the detached plate, called palatine by Dr. Henle, be not rather the ento-pterygoid. The hyoid arch in the Squatina, as in most other Plagiostomes, consists of two long and strong lateral pieces or cerato-hyoids (cornua of Anthropotomy), and a median flattened symmetrical piece, the basihyoid, (corpus ossis hyoidei) below. Short cartilaginous rays extend outwards from the back part of the cornua, supporting the outer membranous wall of the branchial sac: these answer to the branchiostegal rays in osseous fishes, and support the diverging appendage or limb of the hyoidean arch. But the fold of integument in which they project is not liberated, and is continuous with that supported by the opercular rays from the tympanic pedicle. Five branchial arches succeed the hyoidean; but are suspended, as in the Lamprey, from the sides of the anterior vertebræ of the trunk. The Cestracion, so interesting from its early introduction into the seas of this planet, is not so far advanced in cranial development as is the more modern Squatina. In the existing species of the Australian seas (Cestracion Phillipi, v. pl. 10.), the cartilaginous basi-occipital retains a deep conical excavation, adapted to a corresponding one in the atlas, which cavity is consolidated by cartilage in the Squatina ; the original place of the extended anterior end of the chorda, along the middle of the posterior half of the basi-cranial cartilage, continues membranous, and the pituitary perforation is permanently closed by membrane only; the basal cartilage expands anterior to this, and comes into close connection with the maxillary arch, and is thence continued forwards, contracting to a point between the nasal capsules, which meet at the middle line above the symphysis of the upper jaw. The proper cranial cartilage is thinner than in the Squatina; the anterior or pineal fontanelle forms an extended membranous tract on the upper part of the cranium; the vertical ridges, which rise from the sides of this tract, extend forwards and outwards to support the nasal sacs, and are continued backwards, interrupted by a notch filled by membrane, to the posterior angular processes, which overhang the joint of the maxillo-hyoidean pedicle. The maxillary and mandibular arches are as simple as in the Squatina, but much stronger, since they support a series of massive grinding teeth, as well as pointed ones or laniaries. The rami of the lower jaw are confluent at the symphysis. The Skates and Rays have the skull movably articulated, as in Squatina, by two basilar condyles and an intervening space, to the axis.* The skull is flat and broad; the upper wall membranous for a greater or less extent, except in Narcine, where it is closed by *The basi-occipital also affords a small but distinct intermediate surface between the two large condyles in the Zygæna. cartilage. The anterior or vomerine part forms a long pyramidal rostrum, to which are articulated cartilages connecting its extremities with the radial or anterior angles of the enormously developed hand (pectoral fin): in the space between the skull and those fins, the Torpedo carries its electric batteries. The tympanic pedicles are short and thick; the maxillary and mandibular arches long and wide, stretching transversely across the under part of the head. In the ordinary Sharks the anterior prolongation of the cranial cavity gives a quite anterior position, and almost vertical plane, to the fontanelle: three columnar rostral cartilages are produced, two from above, and one from between the nasal cavities, which processes converge and coalesce to form the framework of a kind of cut-water, at the fore-part of the skull. In the place of articular condyles, processes extend backwards from each side of the occipital foramen and clasp, as it were, the bodies of three or four anterior vertebræ of the trunk. The pterygoidean arches extend outwards, in Carcharias, from the base of the cranium, but, as in embryo osseous fishes, are confluent therewith at both ends. The maxillary arch, suspended near its closed anterior extremity to the vomerine part of the base of the skull, is thence extended backwards to the articulation of the lower jaw. A simple cartilaginous pedicle forms the upper part (pleurapophysis) of the mandibular arch, which is completed below by the lower jaw. A few cartilaginous rays diverge outwards and backwards from the pedicle, and support a small opercular flap or fin. The hyoid arch consists of a basi-hyoid and two simple ceratohyoid cartilages; the stylo-hyoid is ligamentous, as in the Squatina. Short cartilaginous rays diverge from the cerato-hyoid to support the branchiostegal membrane, or hyoid fin. The scapular arch, which we shall find normally articulated with the occiput in osseous fishes, is attached thereto, at a little distance behind the head, by ligament and muscles in the sharks: from this arch, also, cartilaginous rays immediately diverge for the support of a radiated appendage or fin; the third in the series counting backwards from the tympanic or opercular fin. The capsules of the special organs of sense are all cartilaginous : that of the ear is involved in the lateral walls of the cranium; that of the eye is articulated by a cartilaginous pedicle with the orbit; and the olfactory sacs are over-arched by the nasal processes of the epicranial cartilage. Amongst the stranger forms in which special development radiates, in diverging from that stage of the common vertebrate route attained by the Plagiostomes, may be noticed the lateral transverse elongations of the orbital processes, supporting the eye-balls at their extremity, and giving the peculiar form to the skull of certain Sharks, thence called "Hammer-headed" (Zygana). In the Eagle-ray (Myliobates) a cartilage is attached to the anterior prolonged angle of the great pectoral fin, and connects it with the fore-part of the cranial (internasal) cartilage; it supports a number of branched and jointed cartilaginous rays, which project forwards, and are connected at the middle line with a like series from the opposite side of the head; they may be regarded as partial dismemberments of the great pectorals; and in Rhinoptera Braziliensis their supporting cartilage is directly continued from that of the pectoral fins, though it is closely attached to the fore-part of the head. These form what Müller has termed "cranial fins;" but the parts more properly meriting that name are the opercular and branchiostegal appendages of the tympanic and hyoidean arches. Having traced in the examples of cartilaginous fishes selected for demonstration, the progressive steps by which the typical features of the ichthyic skull are modelled, as by the hand of the sculptor, in the yielding gristle, we have next to consider them with their leading varieties, as they are permanently wrought out in hard bone. We saw that the base of the skull was first formed by the anterior prolongation of the gelatinous chorda dorsalis, and that the cranial cavity resulted from the extension of the membrane from the fibrous sheath of the gelatinous chorda over the anterior end of the nervous axis. We saw next the superaddition of special capsules for the organs of sense; and then the cartilaginous tissue developed from the basis cranii, according to a pattern common to the lowest forms of the class, and to the embryos of the higher forms which the Cyclostomes permanently represent. We saw the cartilaginous tissue acquiring a firmer texture, hardened by superficial osseous grains, or tesseræ, mounting higher upon the lateral and upper walls of the cranium, and at length entirely defending it: and we then also recognised the maxillary, mandibular, and hyoidean arches, established in a firm cartilaginous material, and on a recognisable ichthyic type. We have next to trace the course and the forms under which the osseous material is superadded to, or substituted for, the primitive cartilaginous material of the skull in osseous fishes; and the remarkable transitional genus Lepidosiren, whose organisation I first made known under the name of Protopterus (xxxIII.), offers the most natural and instructive passage in the shape and structure of its skull, between the gristly and the bony fishes. In the Lepidosiren ossification of the cranial end of the chorda dorsalis extends along the under and lateral part of its sheath, backwards to beneath the atlas and axis (fig. 27. 1), the posterior slightly expanded end of this ossified part supporting, as in the Squatina, the neurapophyses of the atlas (fig. 28. n), the bases of which expand and meet above that end of the ossified chorda and below the spinal canal. Skeleton of Lepidosiren annectens. Ossification of the fibrous sheath of the chorda, commencing posteriorly at its under part (ib. b), ascends upon the sides as it advances forwards, and incloses it above, where it supports the medulla oblongata, and the lateral bony plates (neurapophyses) called 28 Atlas and A ex-occipitals (ib. 2); leaving behind a wide oblique concavity lodging the anterior unossified end of the 'chorda,' which does not extend further upon the 'basis cranii.' The ex-occipitals (fig. 27, 28. 2, 2), expand as they ascend and converge to meet above occipital vertebra, Lepidosiren. the foramen magnum' which they complete. small mass of cartilage connects their upper ends with each other, and with the overhanging backward projecting point of the frontooccipital spine (ib. 3). This cartilaginous mass answers to the base of the supra-occipital in better ossified fishes: a similar cartilage connects the ex-occipitals with the occipital spine in the Tetrodon. We clearly perceive in the Lepidosiren that ossification, advancing on the common cartilaginous mould of the plagiostomous skull, has marked out the posterior cranial vertebra, and not only its neurapophyses but also its centrum; the neural spine being left in a less completely ossified state than in the vertebræ of the trunk. The occipital pleurapophyses (scapula, fig. 27. 51) are much more developed, and appear as two strong, bony, styliform appendages, articulated by a synovial capsule and joint, one on each side, to the persistent cartilaginous base of the neurapophyses (ex-occipitals), and partly to the centrum or basi-occipital. To the lower and less expanded ends of the pleurapophyses are attached the extremities of the hæmapophyses (coracoids, fig. 27. 52); and thus is completed the hæmal arch of the occipital vertebra, here unusually developed in relation to its office of protecting the heart and pericardium: the hæmapophyses or coracoids belong to the same category of vertebral elements as the sternal ribs which protect the heart in higher Vertebrata. The costal or hæmal arch of the occipital vertebra of the Lepidosiren supports an appendage (fig. 27. 57), projecting outwards and backwards like |