168 



FISHES. 



pregnated, and the impregnation must take place before they 

 are invested with a tough leathery envelope which would be 



Fig. 80. — Egg-shell of Cestracion philippi, half natural size, linear. 

 I. External view. II. Vertical section. 

 a, One spiral ridge ; h, The other spiral ridge ; c, Cavity for the ovum. 



impenetrable to the semen, that is, before they enter the uterus ; 

 therefore, copulation must take place in all these fishes. The 

 form of the egg-shell differs in the various genera ; generally 

 (Fig. 79) they are flattened, quadrangular, with each of the 

 four corners produced, and frequently prolonged into long 

 filaments which serve for the attachment of the ova to other 

 fixed objects. In Notidcmus the surfaces are crossed by 

 numerous ridges. In Cestracion (Fig. 80) the egg is pyriform, 

 with two broad ridges or plates, wound edgewise round it, 

 the two ridges forming five spires. The eggs of Callorhynclms 



