328 



CHORD ATA. 



to the facets are the three basal elements of the fin, called 

 the propterygtum, mesopterygium and metapterygium^ the 

 first and last extending forwards and backwards. Each 

 bears on its outer border a row of numerous fin-rays which 

 are jointed. 



The posterior elements are tho, pelvic girdle diXid pelvic fin. 

 The girdle consists of a ventral pubic bar at each end of 

 which is a small dorsal iliac process^ an anterior prepubic 

 process and a pair of acetabular facets. To the posterior of 



Fig. 234. — Dorsal View of Pelvic Girdle and Fins of the 

 Skate. {Ad fiat. ) 



Pubic Bar. Prepubic Process. 



J\ First Fin-ray. 



Iliac Process. 



Metapterygium. 



Distal Joint of 

 Basi pterygium. 



Second and Third - 

 Fin -rays. 



these is articulated the metapterygium, bearing a number of 

 fin-rays. To the anterior facet is attached the thickened 

 first fin-ray. 



In its complete adaptation to an aquatic habitat, with gills 

 or gill-slits, its paired fins with fin-rays, its two-chambered 

 respiratory heart, and its sensory ampullae, the skate is a type 

 of its class Pisces. 



In its cartilaginous skeleton, its placoid scales, heterocercal 

 tail, spiral intestine and cloaca, the form of its urogenital 

 organs and its embryonic development, it is typical of the 

 order Elasmobranchii. 



Its dorso-ventrally compressed body and reduced dorsal 

 fin are typical of the J^atoidei, a group comprising the rays 

 and skates. Minor f«.atures determine its family and genus. 



