306 • MANUAL OF THE MOLLTJSCA. 



{lUtorinm), 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 sessilo 

 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. 





Fig. 134. Lingual teeth of Cydophorus.] 



The sexes are distinct ; the shell is spiral, and closed by an 

 operculum, presenting many beautiful modifications of structure 

 characteristic of the smaller groups, vrhich are often peculiar 

 to limited regions, as in the Helicidce. The oldest fossil species 

 are found in the Eocene Tertiary. 



Family VI. — CycLOSTOMiD^. 



Shell spiral, rarely much elongated, often [depressed, sj^irally 

 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. 



Cyclo STOMA, Lamarck. 



Etymology, Cydos, 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 ^ 

 very thin. Operculum shelly, pauci-spiral. :| 



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



cijclostomidce are conti-actile only. « 



t C. aquilum. Shy. {ongmaX). From a specimen gathered by J. W. Laidiay, Esq., ; 



on the steps of the great idol-temple of Mouhnein, Bjrmah. 5 



