xiri 



l'll\l.l AI ("IIORDATA 



k;*.) 



is a series of intcrcalaiy cart liaises. Thocandal fin is near!}- always 

 heteroccrcal. Clasj)ors arc always developed. A distinct opercular 

 fold is never present. 



¥i<i. SJI).— Acanthodes wardi. (Kestorcd, after Duan.) 



Snh- Order a. — Protoselachii. 



Selachii in which the spinal column is uncalcified, and the centra 

 are very imperfectly developed ; there are more than five branchial 

 arches. Except in Chlamydosdachus, the palatoquadrate develops 

 a process by which it articulates with the postorbital region of the 

 skull. 



This sub-order includes the Nolidanidce {Hexanclms and Heptan- 

 chus), and Chlamydosdachus (Fig. 830), as well as, probably, many 

 fossil forms. 



Fig. 830.— Chlamydosdachus anguineus. (Fmui the Canibndye Xatund Htstori/, after 



Giuithcr.) 



Sub-Order h. — JEuselachii. 



Selachii in which the spinal column is partly or completely 

 calcified. There are only five branchial arches. The palatoquad- 

 rate has no postorbital articulation with the skull. 



Sedion a. — Squalida. 



Euselachii with fusiform body and well-developt'd caudal fin. 

 The j3ectorals are of moderate size. A ventral fin is present. The 

 vertebra? of the anterior part of the spinal column are not fused 

 together. The branchial apertures and the spiracle are situated 

 laterally. 



This section comprises all the recent Sharks and Dog-fishes, with 

 the exception of the Protoselachii. 



