164 



ZOOLOGY 

 ORDER 4. SELACHII. 



SECT. 



Living and extinct Elasmobranchs in which the skeleton of the 

 paired fins is never of the nature of an " archipterygium." The 

 notochord is more or less completely replaced by vertebrae, and there 

 is a series of intercalary cartilages. The caudal fin is nearly always 

 heterocercal. Claspers are always developed. A distinct opercular 

 fold is never present. 



FIG. 840. Acanthodes wardi. (Restored, after Dean.) 



Sub-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 Chlamydoselachus, the palatoquadrate develops 

 a process by which it articulates with the post-orbital region of the 

 skull. 



This sub-order includes the Notidanidce (Hexanchus and Heptan- 

 chus), and Chlamydoselachus (Fig. 841), as well as, probably, many 

 fossil forms. 



FIG. 841. Chlamydoselachus anguineus. (From the Cambridge Katura i History, after 



Giinther.) 



Sub-Order b. Euselachii. 



Selachii in which the spinal column is partly or completel 

 calcified. There are only five branchial arches. The palatoquad 

 rate has no post-orbital articulation with the skull. 



Section a. Squalida. 



Euselachii with fusiform body and well-developed caudal fin. 

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

 vertebrae of the anterior part of the spinal column are not fused 



