EXTERNAL CHARACTERS. 219 



during its development a supply of nutriment direct from 

 the mother. 



I. EXTERNAL CHARACTERS. 

 Lay the dog-fish on a board, and determine the following 



A. General Appearance. 



The body is greatly elongated : it is broadest about the 

 level of the pectoral fins, in front of which it is flattened 

 dorso-ventrally, ending in the blunt rounded snout. The 

 hinder part of the body is compressed laterally, and tapers 

 gradually to the end of the long slender tail, the hindmost 

 three or four inches of which are bent up at an angle. 



The general colour is grey ; paler on the ventral surface, 

 and marked on the dorsal and lateral surfaces with dark 

 brown spots which are generally larger and fewer in the female 

 than in the male. The animal is capable of slightly altering the 

 intensity of these spots. The entire surface is covered with 

 small scales, and along each side of the head and body there 

 runs a slight longitudinal mark, the lateral line. 



B. The Fins. 



The fins are flattened outgrowths supported by an internal 

 cartilaginous skeleton, and usually triangular in shape, the 

 base or attached border being very commonly free at its 

 posterior end. 



1. Median Fins. 



a. The dorsal fins are two in number, the anterior 



and larger one being placed about the middle of 

 the length of the body, and the posterior one 

 a short distance further back. 



b. The caudal fin forms a vertical fringe round the 



posterior four or five inches of the tail. Its dorsal 

 portion is narrow, its hinder end truncated, and 

 its ventral portion partially subdivided into a 

 small posterior and a large anterior lobe. 



