OF MOLLTJSCA. 55 



part of the back of the foot, short, truncated, sickle-shaped, thin and 

 pellucid, formed of somewhat concentric striae ; the inner front edge 

 is thickened, and the whole of the inner edge, together with the lip, 

 is furnished with a thin membranaceous expansion, which is much 

 broader at the two ends. The use of this thin edge is doubtless to 

 enable the animal to adapt its operculum more exactly to the shape 

 of the mouth of the shell. It has been observed that shells with 

 a toothed columella never have opercula, but many instances may be 

 adduced to the contrary. Trochus Pharaonis is not distinguished 

 in this respect from the other shells of that genus, and the genera 

 Turbinellus, Fasciolaria and Pyramidella are all furnished with 

 opercula. — Gray, Spic. Zool. ii. 183. This description of the ani- 

 mal was from a specimen long kept in spirits. Mr. Alder calls the 

 tentacula the disk. 



" Tornatella. Animal Bulla est, et ab illo B. striates parum differt. 

 Pes oblongus, testa parum longior, antice truncatus angulis recurvis 

 uncinatis, postice obtusus, supra operculum parvum ; apertura longe 

 minus, oblongum, corneum gerit, qui striis incrementi cum Murieum, 

 &c. operculo convenit. Caput depressum, sulco profundo a pedo 

 divisum, cum tentaculis coalitum, antice paullo dilatatum, medio 

 excisum, superne sulco mediano longitudinali divisum, postice in 

 lobos duos acutos, lanceolatos, supra testam usque ad medium ejus 

 reflexos terminatum. Oculi duo parvi in medio capite. Color ani- 

 malis albus."— Phil. Sicil. ii. 143, 1844; Wiegm. Arch. 1841, 55. 

 t. 5. f. 10 (and Chiaje, t. 77. f. 13). 



The animal of this genus {Acteon) comes very near to that of 

 Bulla, having a broad lobed disk in front, without tentacles. We 

 find too that the branchial aperture is lateral, the cloak being closed 

 across the front, and open only on the right side, which further 

 confirms us in assigning it a place in this Order (Tectibranchiata) . 

 The operculum can scarcely be considered of more than generic 

 importance. — Alder, Cat. Moll. N. 29. 



2. Solidula. 



Shell thick, solid, ovate, spirally grooved ; spire conic, acute ; 

 aperture longitudinal, narrow, entire, and rounded in front; inner 

 lip callous ; pillar with two plaits, the anterior grooved and continued 

 into the outer lip. Operculum transverse, elongate, curved, imbri- 

 cate ; scar linear. Animal ? 



1. S. nsevia. 



3. MONOPTYGMA. 



Shell subulately turrited, spirally striated ; apex simple, acute ; 

 aperture oval, larger than wide, rounded and entire in front ; colu- 

 mella with a single oblique fold. 

 1. M. striata, Gray, P. Z. S. 1851, 222. 



It differs from Acteon in being elongated, and in having an oblique 

 fold instead of a transverse plait on the columella. — Adams. 



