510 The Class Elasmobranchii or Shark-like Fishes 



perfectly segmented vertebrae are joined in pairs, each pair 

 having two neural arches. In the Asterospondyli or ordinary 

 sharks each vertebra has its calcareous lamella radiating star- 

 like from the central axis. In the Cyclospondyli (Squalidce, 

 etc.) the calcareous part forms a single ring about the axis, 

 and in the Tectospondyli (Squatina, rays, etc.) it forms several 



FIG. 299. Section of vertebras of sharks, showing calcification. (After Hasse.) 

 1. Cyclospondyli (Squalus); 2. Tectospondyli (Squatina); 3. Asterospondyli 

 (Carcharias). 



rings. These groups again are natural and correspond fairly 

 with those based on other characters. At the same time 

 there is no far-reaching difference between Cyclospondyli and 

 Tectospondyli, and the last-named section includes both sharks 

 and rays. 



Nothing is known of the Polyospondyli, and they may never 

 have existed at all. The Diplospondyli do not differ very 

 widely from the earlier Asterospondyli (Cestraciontes) which, as a 

 matter of fact, have preceded the Diplospondyli in point of 

 time, if we can trust our present knowledge of the geological 

 record. 



Other Classifications of Elasmobranchs. Characters more fun- 

 damental may be drawn from the structure of the pectoral 

 fin. In this regard four distinct types appear. In Acanthoessus 

 this fin consists of a stout, stiff spine, with a rayless membrane 

 attached behind it. In Cladoselache the fin is low, with a very 

 long base, like a fold of skin (ptychopterygium), and composed 

 of feeble rays. In Pie ur acanthus it is a jointed axis of many 

 segments, with a fringe of slender fin-rays, corresponding in 

 structure to all appearance to the pectoral fin of Dipnoans and 



