PISCES FISHES. 



535 



therefore, separates the valvular lips of the opening, and no sooner 

 has the little Shark thus extricated itself from its confinement than 

 the two sides close again so accurately that the fissure is not at all 



perceptible.* 



Fig. 237. 



* According to Cuvier, in those Sharks which are viviparous, that is, whose young 

 are hatched in the oviduct prior to their expulsion, this egg-shell is never formed, 

 and the investments of the fetus remain permanently membranous. Loc. cit. p. 397. 



