492 YOUNG OF CYPRJBA. 



namely, a very simple spiral helicoid shell, and an animal 

 furnished with two ciliated wings or lobes, by which it can 

 swim freely through the fluid in which it is contained."* 

 This forms the nucleus of the Cowry shell, which afterwards 

 grows and undergoes several changes in form, gradually 

 becoming more and more complicated until the outer lip is 

 inverted and marked with numerous sulci. The converse 

 of this, however, would appear to take place in other 

 Gasteropoda, as shown in the development of Dolabella, 

 Aplysia, and others, where the shell at first turbinated 

 and nautiloid in shape, afterwards becomes an internal, 

 flattened, horny plate. On placing the young Cowries 

 in a watch-glass of sea-water, they may be seen to 

 whirl about like the Hyalaea and Cleodora, and, like 

 Atlanta, to adhere when fatigued to foreign bodies, not 

 indeed by any sucking disc, but by means of the dilated 

 expansion of their mantle. In the course of growth these 

 fleshy expansions become entirely absorbed, and do not 

 ultimately constitute the lobes of the mantle which em- 

 brace and partially cover the shell in the adult. It would 

 constitute an interesting enquiry to observe the transi- 

 tions in the figure of the animal and shell throughout the 

 entire series of Molluscous groups, as I am convinced that 

 many phases exhibited in their metamorphoses would 

 throw new light not only on the identity of species, but 

 on the reality of the existence of certain genera. 



Rostellaria has all the habits of the Strombidce, pro- 

 gressing by means of its powerful and elastic foot which 

 it places under the shell in a bent position, when sud- 

 denly, by a muscular effort, it straightens that organ, and 



* Edin. Phil. Journal, xxxvi. p. 326. 



