306 



M\NT7AL OF TEE MOLLUSCA. 



{Uttorince), and chiefly differ from tliein 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. 





134. Lingual teeth of CyclophoritsA 



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 HeUcidce. The oldest fossil species 

 are found in the Eocene Tertiary. 



Family VI. — Cyclostomidje. 



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 



Cyclostoma, Lamarck. 



Etymology, Cyclos, circle, stoma, mouth. 



Type, C. elegans, PI. XII., Fig. 40. 



Synonym, Leonia (mammillaris) and Lithidion, Gray. 



Shell turbinated, thin, axis perforated; aperture oval; 

 peristome continuous, simple, straight or expanded ; epidermis 

 very thin. Operculum shelly, pauci-spiral. 



* The tentacles of the helicidee are retractile by inversion (p. 18), those of the 

 ojclostomidte are conti-actile only. 



t C. aquilum,Sby. (original). From a specimen gather;'d by J. W, Laidlay, Esq., 

 on the steps of the great idol-temple of Moulmein, Birmali. 



