UNIVALVES. — BULLA. 87 



BULLA. — Dipper or Bubble. 



Animal — a Limax: Shell univalve, convolute, unarmed 

 with teeth: aperture a little straitened, oblong, longitu- 

 dinal, very entire at the base; pillar oblique, smooth. 

 THIS genus, which contains fifty-five species, is in some 

 instances so nearly allied to the preceding, that much cau- 

 tion is necessary to prevent confusion in the classification ; 

 so great indeed is the difficulty of distinguishing the young 

 shells of the Bulla and Cj^rsea, that the two genera have 

 been intermixed by some authors. However, one grand 

 mark of distinction in this genus is, that, in whatever stage 

 of growth its species are found, they never have teeth on 

 both their lips, the pillar-lip being invariably free from 

 any appearance of denticulations ; while in the Cyprsese, 

 both the pillar and outer lips are crenated with very articu- 

 late and prominent teeth. There is also a greater variety 

 of form in the Bullae than in the CypraeEe. Some species, 

 as the B. volva, or weaver's shuttle, are of an elongated 

 form, having the length much increased by two produced 

 beaks. 



The next variation of form is discernible in the B. ovum, 

 or poached egg, of which there are two varieties : the com- 

 mon, from Amboyna, is white without, and yellow within ; 

 the rarer, from the Friendly Isles, is white without, and 

 pink within. These shells are less beaked and more gib- 

 bous than the B. volva, and lead into the following orbicu- 

 lar species, viz. the B. naucum, B. physis, B. ampulla, &c. 

 These are without teeth, and rather umbilicated. 



The B. terebellum is an exception to the general form of 

 the Bullae, its shape being remarkably long and slender, 

 and resembling a lengthened olive. 



