68 FISHES. 



the difference in their texture, they appear in the form of 

 an X.^ The interspaces between the neurapophyses of the 

 vertebrae are not filled by fibrous membrane, as in other 

 fishes, but by separate cartilages, laminm or cartilagines 

 ■inter crur ales, to wliich frequently a series of terminal pieces 

 is superadded, wliich must be regarded as the first appear- 

 ance of the interneural spines of the Teleostei and many 

 Ganoids. Similar terminal pieces are sometimes observed on 

 the ha?mal arches. Bibs are either absent or but imperfectly 

 represented (Carcliarias). 



The substance of the skull of the Chondropterygians is 

 cartilage, interrupted especially on its upper surface by more 

 or less extensive fibro-membranous fontanelles. Superficially 

 it is covered by a more or less thick chagreen-like osseous 

 deposit. The articulation with the vertebral column is 

 effected by a pair of lateral condyles. In the Sharks, be- 

 sides, a central conical excavation corresponds to that of the 

 centrum of the foremost vertebral segment, whilst in the 

 Bays this central excavation of the skull receives a condyle 

 of the axis of the spinous column. 



The cranium itself is a continuous undivided cartilage, 

 in which the limits of the orbit are well marked by an 

 anterior and posterior protuberance. The ethmoidal region 

 sends horizontal plates over the nasal sacs, the apertures of 

 which retain their embryonic situation upon the under 

 surface of the skull. In the majority of Chondroptery- 

 gians these plates are conically produced, forming the base 

 of the soft projecting snout ; and in some forms, especially 

 in the long-snouted Eays and the Saw-fishes (Pristis) this 

 prolongation appears in the form of three or more tubiform 

 rods. 



1 C. Hassc has studied the modifications of the texture of the vertebrae 

 and the structure of the Chondropterygian skeleton generally, and shown that 

 they correspond in the main to the natural groups of the system, and, conse- 

 quently, that they offer a valuable guide in the determination of fossil remains. 



