0^ INTRODUCTION. 



it may, we have little doubt that in the Atuihleps 

 t'^trophthahnus^ one of the family, this peculiarity 

 exists. It is so distinguished by Cuvier : as is also 

 the genus Poecilia^ a confined group of small fi*hes 

 which inhabit the fresh waters of America (Cuv. 

 & Val., xi. 334) ; and also the Silures (lb. i. 393), 

 which may therefore be added to the list. 



Upon the whole, therefore, this mode of deve- 

 lopment is rare in Osseous fishes, whilst the reverse 

 is the case in Cartilaginous ; the sharks and rays, 

 for the most part, belonging to this division. Of 

 the sharks, we name the families GaleL Mttsteliy 

 ZygceihT. Ahpecir. Spinaces^ Scymniy and Sp.ta- 

 titio'; and of the Rays, the families Prist ide^s Bhi- 

 nobatides^ Torpedines, Try^jjues, Mylio^'^.Uide.^, and 

 Cephalopter'jF. The coverings of the ovum in these 

 ovo-v^viparous fishes are remarkably thin ; and the 

 ova increase in size, as previously hinted, by the 

 absorption of the surrounding fluid. Dr. Davy hav- 

 ing observed that a developed embryo of the Torpedo 

 is much heaAner than an undeveloped one. In one 

 instance, before the appearance of the embryo, the 

 ovum of a torpedo weighed 182 grains — an ovum, 

 in which the embryo was visible, 177; whilst the 

 weight of the mature fish, preWous to birth, was 4'9 

 grains; a fact which is important, as it shows how 

 nearlv allied are the viviparous development with- 

 out immediate connexion with the parent, and the 

 viviparous development in which that connexion 

 su^»sists. 



-' third method of development exists oidy in 



