74 UNIVALVES. 



species (Bulla Ovum) bears a very striking re- 

 semblance to the Cypraea, it differs chiefly 

 from that genus by the absence of teeth on 

 the columellar lip. Some of the Bullae are 

 remarkable for having both ends of the mouth 

 produced into long beaks ; these shells are called 

 birostrate.* (Bulla Volva.) f 



The inhabitants of many of the Bullae are 

 larger than their shells, in consequence they 

 cannot wholly recede into them ; indeed in many 

 the usual order is reversed, and the shell is so 

 surrounded and enveloped in the body of the 

 animal, that no part of it is visible, and an in- 

 experienced person would little expect to find a 

 regular testaceous specimen buried in an unsightly 

 slug. 



A very remarkable circumstance in the animal 

 of the Bulla, is the form and structure of the 

 stomach, which is furnished with a gizzard ; this 

 organ is composed of three strong shelly pieces, 

 which have a roughness and prominency in their 

 centre, that fits them to grind whatever comes 

 under their action ; these plates are surrounded 

 and connected by a cartilaginous ligament by 

 which they are moved. It is supposed that such a 

 structure is necessary to the existence of these 

 mollusks ; for they are exceedingly voracious, 

 as is evident from the fact that the animal of 

 Bulla aperta has been found quite distorted from 



* The children being acquainted with the words rostrum 

 and bis, will see at once the derivation of this word, 

 t Plate II. fig. 8. 



