48 CLASS III. GASTEROPODA. ORDER IV. PLEUROBRANCHIATA. 
included many species of very anomalous construction, such, for instance, 
as are now referred to the genera Achatina, Pyrula, Ovulum, Terebellum, 
&c.; Bruguiére, however, may be said to have established a complete re- 
formation, retaining it for the reception of only those thin fragile bubble 
shells of which the Bulla fasciata or lignaria may be regarded as the type. 
The genus Bullea of Lamarck, Scaphander of De Montford, was proposed 
for the purpose of distinguishing the Bulla aperta, on account of its shell 
being very slightly convoluted, and entirely concealed within the mantle. 
But De Roissy, in adopting this genus, included the Bulla lignaria, in 
consequence of a peculiarity which it has in common with that species, 
namely, the internal structure of the stomach, which in both cases is 
protected with three shelly plates. Another genus was also proposed by 
De Férussac, with the appellation of Bullina, for those species in which 
the edge of the frontal disc is produced into lobes, and whose shells have 
prominent spires ; the Bulla naucum, too, is the genus Atys of De Mont- 
ford. 
The shell of Bulla may be described as being thin, sometimes very fra- 
gile, ovate or subcylindrical, smooth, and lightly convoluted ; the spire is 
short, depressed or concave, and rarely prominent ; the aperture is large, 
sometimes very open, and the outer lip is generally a little inflected 
towards the apex. 
Examples. 
Pl. CLIIL. Fig. 1. 
Butta FasciatTa, Bruguiére, Dict., No.15. Enc. Méth. pl. 359. f. 1. a, 6. 
Chemn. Conch., vol. x. pl. 146. f. 1348 and 1349. 
Bulla velum, Gmelin. 
Pl. CLIT. Fig. 2. 
Butta apLustre, Linneus, Syst. Nat. p. 1184. Enc. Méth., p. 359. 
f.2.a,b. Chemn., Conch., vol. x. pl. 146. f. 1350 and 1351. 
Aplustra —? Schumacher. 
Pl. CLIT. Fig. 3. 
Buixa aperta, Linneus, Syst. Nat., p. 1183. Chemn. Conch., vol. x. 
