320 A HISTORY OF FISHES 



a rudimentary capsule is at first formed round the egg, this 

 being later absorbed. The eggs are large and filled with yolk, 

 and fertilisation takes place in the upper part of the oviduct. 

 In the oviparous forms, the eggs, as they pass down the oviducts, 

 are enclosed in a shell or envelope of horny texture, tough but 

 not brittle, and of a flattened, oblong shape, which besides the 

 egg, contains a certain amount of semi-fluid, albuminous 

 material. The capsule is formed by a special gland peculiar 

 to Selachians, situated in the oviduct, and it varies somewhat 

 in pattern according to the species. In Sharks {Pleurotremata) 

 the outer surface may be quite smooth, or delicately ribbed 

 (Fig. 1 1 6a), and the four corners are usually drawn out into 

 long tendrils, which become coiled round pieces of seaweed, 

 rocks, stones, and other fixed objects, and serve to anchor the 

 tgg during development. They may also assist the extrusion 

 of the capsules themselves, for as these project from the oviducts 

 through the cloaca of the female fish, the tendrils become 

 entangled with objects on the sea-floor and thus help to pull 

 the eggs out. As might be expected where such elaborate 

 precautions for the safety of the embryos are taken, the eggs 

 are always few in number, and are deposited one or two at a 

 time over a long period. The Bull-headed Sharks {Heterodontidae) 

 of the Pacific produce eggs of unique shape, these being of 

 relatively large size, and protected by an elongate cone-shaped 

 capsule with very thick walls, provided with two broad flat 

 flanges twisted spirally round it, and two long, coiled filaments 

 at the pointed end (Fig. i i6b). The Greenland or Sleeper Shark 

 (Somniosiis) , alone among Selachians, produces small eggs, and 

 these are deposited in the sea quite unprotected by a horny 

 envelope. 



The oblong capsules of the Skates and Rays {Raiidae) are 

 essentially similar to those of the Sharks, but instead of the 

 coiled tendrils, the corners are produced to form more or less 

 stiff, pointed horns (Fig. ii6c). These are known variously as 

 "skate barrows," "sailors' purses," "mermaids' purses," and 

 "mermaids' pin-boxes," and may frequently be picked up on 

 the shore after storms. The horns of the capsule are hollow 

 and provided with small slits, through which a current of water 

 passes to the contained embryo. The period of incubation 

 lasts from four and a half to nearly fifteen months, and the little 

 fish finally makes its escape through a slit in one end of the 

 capsule, as in the Sharks. The capsules vary in shape and size 

 in the different species, the largest being one hundred and 



