174 MANUAL OF THE MOLLUSCA. 



Animal with short, tapering and. rather compressed tentacles ; foot di- 

 vided transversely into two portions, advanced successively in walking. 



Distr. W. Indies, Europe. In salt-marshes and on the sea-shore. The 

 British species have thin ovate shells, with the spire moderately produced, 

 and the aperture oval. They form the sub-genus Alexia. (denticulata) 

 Leach. Fossil. Eocene. Brit. France. 



CAEYCHIUM, Miiller. 



Type, C. minimum, PI. XII. fig. 39. 



Syn. Auricella, Hartm. 



Shell minute, oblong, finely striated transversely ; aperture oval, toothed, 

 margins thickened, united by callus. 



Animal with 2 blunt, cylindrical tentacles ; eyes black, sessile, near to- 

 gether, behind the tentacles. 



Distr. 3 sp. Europe; N. America. At the roots of grass in damp 

 places, especially near the sea. 



Fossil. Miocene . Europe. 



The genus Siphonaria, described at p. 155, is supposed to be pulmoni- 

 ferous, and to bear somewhat the same relation to Auricula, that Ancylus 

 does to Limnaea. The lingual dentition is similar to Auricula ; the centre 

 teeth are distinct, the laterals numerous and hooked. 



Fig. 100.* 



SECTION B. OPERCULATA.* 



The Operculated land-snails are exceedingly like periwinkles (litorinai), 

 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.J 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. 



* Siphonaria sp. from the Cape; three rows of teeth, c central, / laterals, from a 

 preparation by J. W. Wilton, Esq , of Gloucester 



t Phanero-pnewmona (open-lunged), Gray. The account of this group is chiefly 

 taken from the Catalogue prepared by my friend Dr. Baird. 



J The tentacles of the helicida are retractile, by inversion (p. 25) those of the 

 cyclostomidce are contractile only. 



