CYCLOSTOMIDiE. 2T9 



truncated ; aperture elliptical, peristome continuous, thickened, 

 duplicate, subacute within, subvaricose without. Operculum 

 corneous, subspiral. 



Rostrum elongated, transversely corrugated, emarginate in 

 front ; tentacles very short, triangular, depressed, the 63^68 at 

 their bases ; foot large, divided by a transverse sulcus. 



AcicuLA, Hartmann. 



Si/n. — Acme, Hartmann. Pupula, Agassiz, Auricella, Jurine. 



Distr. — 22 sp. Europe, No. Africa. A spectahilis, Rossm. 

 (Ixxv, 33). 



Shell minute, slender, nearly imperforate ; peristome slightly 

 thickened, margins subparallel, joined by a thin callus. Oper- 

 culum very thin, transparent, paucispiral. 



Family CYCLOSTOMID^. 



Shell spiral, rarely much elongated, often depressed, spirally 

 striated; aperture nearly circular; peristome simple. Oper- 

 culum distinctly spiral. 



Animal with the eyes on slight prominences at the outer 

 bases of the tentacles ; tentacles contractile only ; foot rather 

 elongated. 



Teeth recurved, hooked, in seven rows, arranged in a semi- 

 lunar manner on a narrow lingual band (xi,21). Mouth probos- 

 cidiform, not provided with horny jaws. Respiratory organ 

 reticulate, in the form of a sacciform cavity on the back of the 

 neck ; edge of the mantle free from the nape, leaving the respi- 

 ratory cavity open. Sexes distinct. Oviparous, for the most 

 part terrestrial, and respiring free air. 



The tentacles are simply contractile, and not retractile by 

 inversion as in the Helicidas, and the e3^es are usually sessile on 

 the head near the bases of the tentacles, instead of being elevated 

 on peduncles as in that family. , 



The animal of the Cyclostomidge is very like that of the 

 periwinkle (Littorina), differing chiefly in the situations it inhabits 

 and the medium respired. The operculum presents many 

 beautiful modifications of structure characteristic of the smaller 

 groups, which are often peculiar to limited regions, as in the 

 Helicidse. The oldest fossil species are Eocene. 



I. Pomatiasinse. 

 PoMATiAS, Studer. 

 Distr. — 80 sp. So. Europe, several in N. Africa and India. 

 P. obscurus, Lam. (Ixxv, 38). 



Shell slender, turreted, longitudinally striated ; peristome 

 subcontinuous, reflected. Operculum cartilaginous, paucispiral, 

 composed of two plates and concamerated between them. 



