XVII ELASMOBRANCHII 433 
As a rule each egeg-case has but a single egg, but in Rhinobatus 
and Z'rygonorhina (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 Rays are 
viviparous, that is, the young are 
born alive; the rest, lke the 
Scyllidae (e.g. the common British 
Dog-Fishes, Sceylliwm canicula and 
S. catulus), the Heterodontidae, 
and the Ratidae 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- Fl. 24. pg Dee case ok ee eee 
4 : (Cestracion) galeatus. (From Parker 
ing filaments. When extruding and Haswell, after Waite.) 
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 Seylliwm 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 2F 
