CH. VIl] SNAILS AND SLUGS 235 



BRITISH LAND AND FRESH- WATER GASTROPODS 1 . 



Class Gastropoda. 



Sub-class ANISOPLEURA. 



Order STREPTONEURA. 



Sub-order PECTINIBRANCHIA. PROSOBRANCHIA. 



Basommatophora. Eyes to inner side of cephalic tentacles, at 

 their base; male and female generative apertures separate, 

 on right side of neck. Aquatic. 



Family VALVATID^. Shell turbinate or discoidal ; spire short and 

 symmetrical, aperture round and entire ; umbilicate ; oper- 

 culum horny ; apex central. Aquatic. 



genus Valvata Mull., cristata Mull., |", whorls 5, shell a flat coil.. 

 piscinalis (Mull.), J", whorls 6, shell a broad cone, occasionally in 

 brackish water. 



Family VIVIPARTD^E, genus Vivipara Montf. (Paludina}. Shell 

 turbinate with long symmetrical spire, whorls rounded ; aper- 

 ture oval ; operculum horny and irregularly concentric. Aquatic. 

 contecta (Millet), ^", whorls 7, much rounded ; suture deep, 



umbilicus deep. 



vivipara (L.), 1^", whorls 6^, rather convex ; suture shallow, um- 

 bilicus very slight. 



genus Bithynia Leach, eyes sessile ; operculum calcareous. Aquatic. 

 leachii (Shepp.), J", whorls 5, aperture nearly round. 

 tentaculata (L.), |", whorls 6, aperture oval, angular above. 



1 The nomenclature here adopted is that given in the list of British 

 Land and Fresh- Water Mollusca compiled by B. B. Woodward and 

 published by the Conchological Society in October 1903. I am indebted 

 to Messrs H. H. Brindley and F. C. Morgan of Cambridge for much kind 

 help and information; and also to Gordon's " Our Country's Shells," 

 for many particulars relating to the shell. Undue prominence is really 

 given to the shell, while the characters of the lingual ribbon (radula) and 

 of the soft tissues have been deliberately withheld. 



