748 



MR. C. TATE REGAN ON 



[June 19, 



are exactly similar in structure and position, and pierce the skin 

 in the same manner. Each spine is pointed and more or less 

 cylindrical, and is hollow and implanted on a process of a large 

 basal cartilage which supports all or most of the series of radials. 

 This cartilage is deep proximally and has the upper edge oblique ; 

 it may be triangular or four-sided, but with the posterior edge 

 much shorter than the anterior one. A precisely similar ai-range- 

 ment has been found in Hyhodus. When the spine becomes 

 rudimentary or is entirely lost, this characteristic ari'angement of 

 the cartilages maybe slightly modified (text-fig. 120, 0, D, E & F, 

 p. 747). 



The cartilaginous supports of the pectoral fin (text-fig. 121) have 

 typically the following arrangement : — The propterygium bears a 

 single radial and excludes the mesopterygium from the edge of 

 the fin. The mesopterygium is well-developed ; it is narrowed 



Text-fig. 121. 



Skeleton of the pectoral fin of Sqttaiina (A), Squalus (B), and Fliotrema (C). 

 (B after Gegenbaur.) 



p, propterygium ; m, mesopterygium ; mt, metapterygium. 



proximally, where it articulates direct with the pectoral arch, 

 and expanded distally, bearing a considerable number of radials. 

 The metapterygium is usually similar to the mesopterygium and 

 bears about as many radials. The radials are simple and mostly 

 formed of 3 segments (Squalidse) or 4 (Squatinidpe) or several 

 (Cestraciontidge). 



