GASTEROPODA TECTIBRANCHIATA. 349 



with its concealed spire and ample aperture, very wide anteri- 

 orly, resembles a loosely rolled lamina, streaked in the direction 

 of its whorls. The stomach of the animal is armed with two 

 large semi-oval osseous pieces, and with a small compressed 

 one(l). 



Bulla ampulla, L.; Martini, I, xxii, 20, 204; Cuv., Ann. du 

 Mus., XVI, 1. The shell oval, thick, clouded with grey and 

 brown; the stomach furnished with three black, very convex, 

 rhomboidal pieces. 



Bulla hydatis, L. ; Chemn. IX, cxviii, 1019; Cuv., Ann. du 



Mus., XVI, 1. Shell round, thin, and semi-diaphanous; the last 



whorl, and consequently the aperture, higher than the spire; 



three small scutelliform pieces in the gizzard(2). 



We reserve the name of Akera, properly so called, Doridium, 



Meek., LoBAKiA, Blainv., for those species which have no shell 



whatever, or only a vestige of one behind, although their mantle has 



its external form. 



A small species, Bulla carnosa, Cuv., Ann. du Mus., XVI, 1; 

 Meek., Anat. Compar., II, vii, 1, 3; Blainv. Malac, pi. xlv, f. 

 3, is found in the Mediterranean. The only armature of the 

 stomach is the mantle; its fleshy oesophagus is extremely thick. 

 A tuberculous species, Doridium Meckelii, Delle Chiaie Me- 

 mor., pi. X, f. 15, inhabits the same sea. The 



Gastropteron, Meek. 



Appears to be an Akera, the margin of whose foot is extended into 

 broad wings, used in natation, which it effects on its back. It has 

 no shell, nor has the stomach any armature; a slight fold of skin is 

 the only vestige of a branchial operculum that is perceptible. 



G. Meckelii; Rosse, Diss, de Pteropodum Ordine, Halse, 1813, 

 f. 11 13; and Blainv., Malacol., pi. xlv, f. 5; or Clio amatiy 

 Delle Chiaie, Memor., pi. ii, f. 1 8. A small animal an inch 

 long and two broad, the wings being extended. From the Me- 

 diterranean. 



(1) Gioeni having' observed this stomach separate from the animal, mistook it 

 for a shell, and made a genus of it, to which he gave his own name (The Tricla 

 of Retzius, Char, Brug.). Gioeni even went so far as to describe its pretended 

 habits. Draparnaud was the first who perceived this mixture of error and fraud. 



(2) Add, Bulla naucum; Bulla physis. Muller describes smaller ones, such 

 as the Akera bullata, Zool. Dan., LXXI, or Bulla akera. Cm. 



