156 ZOOLOGY SECT. 



teeth were few and large. The lateral line was in the form of an 

 open groove. 



FIG. 783. Acanthodes wardi. (Restored, after Dean.) 



ORDER 4. SELACHII. 



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. 



Sub- Order a. Protoselachii. 



Selachii in which the spinal column is unossified, and the 

 centra are very imperfectly developed ; there are more than five 

 branchial arches. The palato-quadrate develops, except in 

 Chlamydoselachus, a process by which it articulates with the 

 post-orbital region of the skull. 



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

 chus), and Chlamydoselachiw, as well as, probably, many fossil forms. 



Sub-Order b. EuselacJiii. 



Selachii in which the spinal column is partly or completely 

 ossified. There are only five branchial arches. The palato-quad- 

 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 

 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. 



Section /3. JRajida. 



Euselachii with dorso-ventrally compressed body, and, usually, 

 feebly developed caudal fin. The pectorals are of great size, the 



