xin PHYLUM CHORDATA 169 



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. 829. 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 Clilamydoselachus, the palatoquadrate develops 

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

 skull. 



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

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

 fossil forms. 



FIG 830 Chlamvdoselaclms anguineus. (From the Cambridge Natural History, after 



GUnther.) 



Sub-Order 1. Euselachii. 



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. 



Section a. Squalida. 



Euselachii with fusiform body and well-developed caudal fin. 

 The pectorals 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. 



