SEXUAL CHARACTERS ELASMOBRANCH FISHES 367 



terior direction. The pseudosiphon is not so large as in Galeus. 

 On the outer side of the rhipidion and between it and the pseudo- 

 siphon is another similar sac, the pera, whose aperture faces 

 forward confluent ^\dth the hypopyle. 



The pera is apparently formed by the folding back of a flap 

 like that of Galeus (which separates the rhipidion from the 

 pseudosiphon) and its fusion with the base of the rhipidion. 

 The use of the pera is problematical: it would appear to be dis- 

 advantageous, since half the spermatozoa which do not flow out 

 on the other side of the rhipidion would enter the pera, and there 

 collect. Wliile vestiges of pseudosiphons may occasionally be 

 seen in Scyllium canicula, no traces of perae can be observed. 



LAMNA CORNUBICA 



The porbeagle 



This animal, coming as it does under the vulgar appellation 

 of 'shark,' presents very peculiar and interesting features, for, 

 in regard to the points I am considering, it approximates not to 

 the Selachoidei, but to the Batoidei. Instead of possessing 

 siphons similar to its congeners, the sac homologous with those 

 structures is small (about 2 cm.) and, as in Raia, is almost com- 

 pletely filled by a gland — the clasper gland. 



The claspers, too, are minute for so large an animal, for, in the 

 porbeagle upon which the following observations were made, 

 captured at Plymouth in July, 1918, and which measured 57 

 cm. from the tip of the snout to the posterior ends of the clasp- 

 ers, they are but 4 cm. in length. Moreover, they are little 

 longer than the pelvic fins, so that they appear to project scarcely 

 at all. Scyllium canicula is the only type under observation so 

 far in which the claspers are not quite as long as the pelvic fins. 

 Further, the clasper groove is not closed, is soft and devoid of 

 dermal denticles, in all these particulars again resembling Raia. 

 The rhipidion is pronounced as in Raia, and accessory structures 

 such as pseudosiphons or perae are absent. The apopyle and 

 hypopyle do not exist as apertures, but merely as local indication 

 (fig. 6). 



