588 Records of the Indian Museum . [Vol. XXII, 



radular teeth in these little molluscs can only be examined under 

 an oil-immersion lens and even at the great magnification such a 

 lens provides do not give an image beyond the possibilities of 

 error in all cases. Moreover, the differences in the minute struc- 

 tures hardly seem to be supported bj' other differences of corre- 

 sponding importance in the anatomy or the shell. We found it 

 very difficult to convince ourselves, for example, that Walker 

 was right in assigning the common Indian Ancylus verruca to Fer- 

 rissta, though there was much less doubt as to the new species 

 here described. We propose, therefore, to retain these species in 

 Ancylus, but to recognize Ferrissia as a subgenus and assign 

 them to it. 



In order to establish their status it was necessary to examine 

 also the named and unnamed collections in the Indian Museum, 

 including the specimens mentioned by Nevill in his " Hand List." 

 Among these we found considerable confusion. As all the species 

 already knov\n from the Indian Empire were represented in the 

 collection, we take this opportunity to revise them here. Shells 

 onl}^ of A. baconi and C. tenuis are available, but we see no rea- 

 son to separate either of these species or A. ceylanicus from the 

 others subgenerically and therefore assign all the Indian species 

 to the subgenus Ferrissia. 



The shells of these species maj? be distinguished by the use 

 of the following key :— 



Key to the Indian species of Ancylus. 



/. Apex oi shell sharply pointed, very liule reflected ; 



shell over 5 inm. long; .. .. A. ceylanicus. 



II. Apex blunt, reflect,;d "to the right: shell less than 5 

 mm. long. 



A. Greatest breadth of shell near the middle; outline 



of shell distincth- asymmetrical bilaterall)- A. vernica. 



B. Greatest breadth of shell in posterior third ; outline 



of shell almost symmetrica! bilaterall}-. 

 i. Sides of shell not parallel ; altitude much less 



than \ of length ,.. ... .-!. baconi. 



ii. Sides parallel ; altitude more than \ of length. 



a. Shell deep violet intcrnallv with minute radi- 



ating striae ... - A. viola. 



b. Shell dull \ello\vish internally, without distinct 



microscopic sculpture ... . A. tenuis. 



Genus Ancylus, Geoffroy. 



1903. Ferrissia, Walker, Nautilus XVII, p. 15. 

 1917. Ferrissia, id., ibid., XXXI, p. 3. 



Although Walker described Ferrissia as a subgenus of 

 Ancylus only in 1903, he raised it to the rank of a subfamily 

 under the name Ferrissiinae in 1917, defining it as follows: — 

 "Jaw segmented in plates. Radula with a bicuspid central, 

 laterals obliquely deflected with from two to five cusps arranged 

 somewhat like the teeth of a comb ; marginals also comb-like, 

 cusps not (usually) extending to the basal line." 



