88 STUDIES ON THE ELASMOBRANCH SKELETON, 



the extremity or have intercalated rays between them ; none of 

 the rest of the rays are distinctly branched. The last five or six 

 rays, which are very short, are supported on a series of irregular 

 basal cartilages. The arrangement of the articulations is such 

 that there are three more or less complete antero-posterior 

 horizontal lines of them ; that situated nearest the base is confined 

 to the last eleven rays ; the middle one stretches completely across 

 the fin from side to side and the distal one, situated not very far 

 from the middle, extends in an irregular line from the seventh ray 

 to the eighteenth. The second dorsal and anal fins are very small, 

 and consist of a few irregular rays without basal plates, and 

 separated by a well-marked interval from the vertebral column. 

 The Caudal Fin. 

 The caudal fin is supported both by supra-vertebral and by sub- 

 vertebral cartilaginous rays, of which the latter are more highly 

 developed than the former. The latter begin a little in front of 

 the upward bend of the spinal column. The first is a short 

 triangular piece articulating movably with two of the hypurals- 

 The following five, which become successively longer, likewise 

 articulate with the hypurals, but the remainder, which gradually 

 decrease in size from before backwards, coalesce with the latter at 

 their bases without the intervention of any articulation. The 

 supi'a-vertebral rays begin a little behind the commencement of the 

 sub-vertebral ; none of them coalesce with the neural arches. In 

 front, where they are more irregular and more or less coalescent 

 with one another, a few of them are separated from the neural 

 arches by a slight interval occupied by fibrous tissue. In many 

 cases a pair of rays may coalesce with one another ; otherwise 

 they correspond in number with the vertebrse. 



HEPTANCHUS INDICUS. 



Plate I., Fig. 5. 



Skull and Visceral Arches. 



The occipital region is characterised by the presence of three 



ridges — a central one continuous with the spinous processes 



