66 



INTRODUCTION. 



it may, we have little doubt that in the Anableps 

 tetrophthalmuS) one of the family, this peculiarity 

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

 the genus Poecilia, a confined group of small fishes 

 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 Galei, Musteli, 

 ZygcenaB, Alopecice, Spinaces, Scymni^ and Squa- 

 tlnoe; and of the Rays, the families Pristides, Rhi- 

 nobatides, Torpedines, Tryc/ones, Myliobatides, and 

 Cephalopterw. The coverings of the ovum in these 

 ovo-viviparous 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 heavier 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, 1 77 > whilst the 

 weight of the mature fish, previous to birth, was 479 

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

 nearly allied are the viviparous development with- 

 out immediate connexion with the parent, and the 

 viviparous development in which that connexion 

 subsists. 



The third method of development exists only in 



