ANATOMY OF VERTEBRATES. 77 



communicates by means of the nervous and vascular foramina 

 with the acoustic chamber in the tliick hiteral wall : this insulation 

 of the labyrinth is common to the Plagiostomes. The cranial 

 ca^'ity is closed by membrane anteriorly. The foramina for the 

 fifth pair of nerves mark the ' alisphenoidal ' portion of the cndo- 

 cartilage : those for the optic nerves the ' orbitosphcnoidal' part: 

 the ' prefrontal' portion is marked by the olfactory foramina, and 

 their articulation Avith the palatine part of the maxillary arch. 



The exterior of the skull is variously and singularly modified in 

 different Sharks and Rays, the developemcnt 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, fig. 

 30, c, articulated to the side of the skull beneath the posterior 

 angular (mastoid) process, has the hyoidean, and partly the man- 

 dibular,' arches attached to its lower end, the former, d, by a close 

 joint, the latter by two ligaments. The maxillary arch, in Squa- 

 tina, is suspended by a ligament from its ascending or palatal 

 process, to the notch between the vomerine and the anterior 

 supracranial cartilaginous plate. From this point the jaw is con- 

 tinued in one direction forward and inward, completing the arch, 

 ib. e, by meeting its fellow, to which it has a close ligamentous 

 junction ; and in the opposite direction, backward and outward, as 

 a coalesced diverging appendage to the outer side of the tympanic 

 jiedicle, where it forms the more immediate articulation for the 

 lower jaw, like the hypotympanic continuation of the upper 

 maxillary bone in the Batrachia, fig. 71, e. Each lateral half or 

 ramus of the mandible, fig. .30, d, consists of a single cartilage, 

 the two being united together at the symphysis by ligament. 



Two slender labial cartilages, ib. f, are developed on each 

 side the maxillary, and one, g, on each side the mandiliular 

 arch ; which complete the sides of the mouth. These cartilages 

 Cuvier regarded as rudmicnts, respectively, of the maxillary 

 and dentary Ijones, the dentigerous maxillary arch as the palatine 

 bones, and the mandit)ular arch as the articular piece of the 

 lower jaw : but both palatines and articulars co-exist with 

 labial cartilages, like those of Squatina, in a Brazilian Torpedo 



' Throughout this work the term 'mandible' is applied to the lower jaw, and the 

 inverted cranial arch which that jaw completes is called ' mandibular ; ' the ai'ch 

 formed by the upper jaw is called 'maxillary.' 



