CYCLOSTOMID AB. 279 
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 eyes at 
their bases; foot large, divided by a transverse sulcus. 
Aciouna, Hartmann, 
Syn.—Acme, Hartmann. Pupula, Agassiz. Auricella, Jurine. 
Distr.—22 sp. Europe, No. Africa. A spectabilis, Rossm. 
(axev, 385): 
Shell minute, slender, nearly imperforate ; peristome slightly 
thickened, margins subparallel, joined by a thin callus. Oper- 
culum very thin, transparent, paucispiral. 
Famity 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 onl 7; 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 Helicide, and the eyes 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 Cyclostomide is very like that of the 
periwinkle (Littorina) differing chiefly in the situations it inhabits 
and the medium respired. The operculuni presents many 
beautiful modifications of structure characteristic of the smaller 
groups, which are often peculiar to limited regions, as in the 
Helicid. The oldest fossil species are Eocene. 
I. Pomatiasine. 
PoMATIAS, Studer. 
Distr.—80 sp. So. Europe, several in N. Africa and India. 
P. obscurus, Lam. (lxxv, 38). 
Shell slender, turreted, longitudinally striated; peristome 
subcontinuous, reflected. Operculum cartilaginous, paucispiral, 
composed of two plates and concamerated between them. 
