3506 MANUAL OF THE MOLLUSCA. 
(Zittorine), and chiefly differ from them in the situations they 
inhabit, and the medium respired. They have a long truncated 
muzzle, 2 slender contractile tentacles, and the eyes are sessile 
on the sides of the head.* The mantle-margin is simple, and 
the pulmonary cavity is situated on the back of the neck, and 
quite open in front. Lingual ribbon narrow; teeth 7-ranked. 
re 
Fig. 134, Lingual teeth of Cyclophorus.t 
The sexes are distinct; the shell is spiral, and closed by an 
operculum, presenting many beautiful modifications of structure 
characteristic of the smaller groups, which are often peculiar 
to limited regions, as in the Helicide. The oldest fossil species 
are found in the Hocene Tertiary. 
FAMILY VI.—CYcLOSTOMIDA. 
Shell spiral, rarely much elongated, often {depressed, spirally 
striated ; aperture nearly circular; peristome simple. Oper- 
culum distinctly spiral. 
Animal with the eyes on shght prominences at the outer 
bases of the tentacles; tentacles contractile only; foot rather 
elongated. | 
CycLosToMA, Lamarck. 
Etymology, Cyclos, circle, stoma, mouth. 
Type, C. elegans, Pl. XII., Fig. 40. 
Synonym, Leonia (mammillaris) and Lithidion, Gray. 
Shell turbinated, thin, axis perforated; aperture oval; 
peristome continuous, simple, straight or expanded; epidermis 
yery thin. Operculum shelly, pauci-spiral. 
* The tentacles of the helicide are retractile by inversion (p.18), those of the 
cyclostomide are contractile only. 
Tt C. aquilum, Shy. (original). From a specimen gathered by J. W. Laidlay, Esq., 
on the steps of the great idol-temple of Moulmein, Birmah. 
