GENUS XXll. BULLA. 103 



\ery various ; the B. Ovum and verrucosa differ but little from 

 the Cypraa, scarcely in any thing- but in the tcclli being want- 

 ing on one lip. The B. Vulva, birostris, Upelta, gibbosa, &c. 

 gradually approach to the form of the Cyprcpa, but have no 

 teeth on either lip. B. Naucuin, aperta (/. 62), Hydatis, 

 Ampulla (^f. 65), lignaria, Physis, Amplustra, &c. are some- 

 what globular in shape, with a large aperture, and the edge 

 of the outer lip very thin and acute ; these may perhaps be 

 considered as the true Bulla, but even these differ considerably 

 fi'om one another. 



B. Ficus (/. 64), Rapa, and those with the straight canal, 

 which is the essential character of the Murex, would seem to 

 arrange better in that genus, where indeed Liimaeus had placed 

 them in his Musceum Ludovicce Ulricce. 



B. fontinaUs, hypnorum, achatina, &c. agree with many 

 species of the Helix. 



B. Terebellum (/. 63) seems more like a Cone ; Linnaeus 

 remarks that it lias the aperture of a Cone. 



What is known of the animals inhabiting these shells proves 

 only the great variety there is among them, and the impro- 

 priety of classing them together. The animals of B. aperta, lig- 

 naria (see pi. 1 1./. 16), Hydatis, &c., and probably of several 

 others, are much larger than the shell, and appear like Mollusca, 

 having the shell concealed under the skin, somewhat similar 

 to the shield of the Laplisia ; these have no tentacula, but two 

 small eyes on the top of the head, and a curious structure of a 



