TUE SKULL OF FISHES. 75 



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 plagios- 

 tomous 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) pro- 

 cess, has the hyoidean, and partly the mandibular *, arches attached 

 to its lower end, the former by a close joint, the latter by two liga- 

 ments. The maxillary arch, in Sqiiatina, 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 Avhich 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 continu- 

 ation 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 maxil- 

 lary, and one on each side the mandibular ai'ch ; which complete the 

 sides of the mouth. These cartilages Cuvier regarded as rudiments, 

 respectively, of the intermaxillary, maxillary, and dqntary 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 S(|uatina, in a Brazilian Torpedo {Narcine), and at the same time 

 with distinct pterygoid cartilages, (xxi. 1835, pi. v. Jig. 3. & 4.) f 



Four or five short cartilaginous rays, in Squatina, diverge from 

 the posterior margin of the tympanic pedicle, and support a mem- 

 brane answering to the opercular flap in Osseous fishes ; in their 

 ultimate homology these rays are the skeleton of the diverging ap- 

 pendage 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." 



t It may be questioned whether the detached plate, called palatine by Ur. Ileiilo, 

 be not rather the cnto-ptery^oid. 



