ELASMOBRANCHII 



433 



a single egg, but in Rhinohatus 



As a rule each egg-case has but 

 and Trygonorliina (Batoidei), both of which are viviparous, each 

 case contains three to four eggs. Generally the egg-cases are 

 somewhat quadrangular in shape, with the four angles, two at 

 each end, prolonged either into short horns, or into long tapering 

 tendrils (Fig. 246). The oval egg-cases of the Heterodontidae 

 are remarkable not only for their size, but also for the presence 



of a broad spiral lamina winding 



round the exterior of the case 



from one pole to the other (Fig. 



245). The majority of the 



Sharks, Dog-Fishes, and Eays are 



viviparous, that is, the young are 



born alive ; the rest, like the 



Scylliidae (e.g. th^ common British 



Dog-Fishes, Scyllium canicula and 



S. catulns), the Heterodontidae, 



and the Eaiidae are oviparous, 



that is, the young are hatched out 



after the extrusion of the eggs. 



In the oviparous species the eggs 



are extruded either singly or in 



pairs, and generally deposited on 



the sea-bottom. When, however, 



the egg-cases are provided with 



tendrils, as, for example, in the 

 two British Dog-Fishes just men- 

 tioned, these organs act as anchor- 

 ing filaments. When extruding 

 an egg, the female swims round 



and round some piece of upright seaweed, and the curling tendrils 

 become entwined round it in such a way that the egg becomes 

 securely attached thereto (Fig. 246).^ The embryos are long in 

 developing, and in Scyllium it may be' several months after 

 fertilisation (200 to 275 days) before they are hatched, the 

 young Fish finally escaping through a rupture in the egg-case. 

 In the oviparous species the nutritive food-yolk stored up, first 

 in the egg and subsequently in the yolk-sac (Fig. 248), suffices 

 for the nourishment of the embryo until the period of hatching, 



1 Cunningham, Marketable Marine Fishes, London, 1896, p. 64. 

 VOL. VII 2 F 



Fig. 245. — Egg-case of Ileterodontus 

 (Cestracion) galeatus. (From Parker 

 and Haswell, after Waite.) 



