306 



MANUAL OF THE MOLLTJSCA. 



{littorince), and cliiefly differ from them in tlie 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 cayity is situated on the hack of the neck, and 

 quite open in front. Lingual ribbon narrow ; teeth 7 -ranked. 



^' 



/f^ 



Fig. 134. Lingual teeth of CydophorusA 



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 Helicidce. The oldest fossil species 

 are found in the Eocene Tertiary. 



Pamilt YI. — Cyclostomld^. 



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. 



Cyclostoma, Lamarck. 



Etymology, Cyclos, circle, stoma, mouth. 



Type, C. elegans, PL XIL, 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 helicidce are retractile by inversion (p. 18), those of the 

 cyclostomida are contractile only. 



t C. aquilum, Sby. (original). From a specimen gathered by J. W. Laidlay, Esq., 

 on the steps of the great idol-temple of Mouhnein, Birmah. 



