80 A Guide to the Zoological Collections 



The skeleton is cartilaginous, though calcification 

 may sometimes occur in parts of it, and the tail is usual- 

 ly heterocercal : the gills are attached to the skin by 

 their outer margins, so that — with the exceptions to be 

 noted — there are as many external as internal gill-open- 

 ings and no gill-cover : there is a muscular conus arterio- 

 sus (see Case 10), with many valves, at the base of the 

 aorta : the mucous membrane of the intestine forms a 

 spiral valve (see Case 10) : the optic nerves do not simply 

 cross each other : impregnation is internal, and though 

 eggs may be laid the young are often born alive 

 (see Case 12): the ventral fins are placed far behind 

 the pectoral fins : the skin may be naked, but more 

 commonly it bears placoid scales, or granules or spines, 

 which last are sometimes hardly distinguishable from 

 small teeth. Geologically, the Chondropterygii are a 

 very ancient order : " some of the earliest determinable 

 fish remains are believed to be, or are, derived from 

 Plagiostomes " (Giinther). 



Order I. PLAGIOSTOMA. 



This Order is divided into two sections which grade 

 into one another, namely : (1) the Sharks [Selachoidei] in 

 which the gill-openings are lateral and the trunk gradu- 

 ally shades into the tail, and (2) the Rays (Baioidei), in 

 which the gill-openings are on the ventral surface and 

 the trunk is more or less disk-like. 



Selachoidei (Sharks). 



Six of the nine existing families of Sharks are repre- 

 sented in Indian Seas. 



Family I. Car char iidce [Cases 39-41]. 

 These are the true Sharks, known of all. The eye 

 has a nictitating membrane. They are common in all 





