928 CLASS PISCES. 



head, with the teeth arranged in numerous rows. The crowns 

 of these teeth present some resemblance to those of the Selachian 

 Hybodus (fig. 865); consisting of one large cone, flanked on either 

 side by one or more smaller cones, of which the outermost is 

 generally the largest. This genus is exclusively Carboniferous, and 

 occurs both in Europe and North America ; a large number of 

 species being known. Dicentrodus, of the Scottish Carboniferous, 

 is distinguished by the teeth having only a single lateral cone 

 developed on one side. Phozbodus, of the Devonian of Iowa, is 

 an allied genus, with the lateral cones of the teeth at least as large 

 as the middle cone ; while in Lambdodus, of the North American 

 Palaeozoic, these lateral cones are totally wanting. In Dicrenodus, 

 of the Carboniferous of both Europe and North America, we have 

 a modification of the Cladodont tooth, in which the central cone is 

 compressed, with the cutting-edges serrated ; while the lateral cones 

 may either be two in number, or absent. Finally, the North 

 American Palaeozoic genus Hypodadodus differs from the preceding 

 by the absence of serrations on the edges of the central cone ; the 

 lateral cones being invariably absent. 



Suborder 2. Selachii. — The existing Sharks, Rays, and their 

 allies are characterised by the endoskeleton being, as a general 

 rule, only superficially calcified ; while, except in some of the earlier 

 forms, the notochord is constricted at the centre of each vertebra. 

 The neural and haemal arches of the vertebrae are short and stout ; 

 and intercalary cartilages are nearly always developed. The pectoral 

 fin (fig. 846) has not a segmented axis, being of the ichthyopterygial 

 type ; and the axis of the pelvic fin of the male is produced into the 

 so-called "clasper," which is connected with reproduction. Some 

 of the other features of this suborder have been already mentioned 

 at the commencement of this chapter. The Selachians may be 

 divided into two sections, according to the structure of the vertebral 

 centra. In the one the anal fin disappears, and there is a tendency 

 to the depression of the body and the enlargement of the pectoral 

 fins, but there is no diminution in the size of the spiracle, and the 

 vertebrae, when fully developed, are of the type known as tectospon- 

 dylic. In the other section the anal fin persists ; the shape of the 

 body is always rounded — not even excessively flattened in the 

 Scilliidce; the spiracle tends to abort, and may be almost or totally 

 absent in the most specialised forms ; while the fully developed 

 vertebrae are of the type known as asterospondylic. The type of 

 vertebral centra known as cyclospondylic occurs in the immature 

 condition of both sections. 



Section a. Tectospondyli. — In this section the vertebral centra, 

 when fully calcified, have the concentric laminae predominating over 

 those that radiate from the centre. The anal fin is invariably want- 



