1921.] Manipur Molluscs. 



M easurements of Shells (in millimetres). 



577 



We figure the radula (fig. 13, B) of a specimen of the inter- 

 mediate phase from Dimapur, Assam. 



Family TLANORBIDAE. 



As the Manipur Valley is rich in species of this family, and 

 as representatives of all the main Indian types occur in it, we 

 have taken the opportunity to carry out, so far as our present 

 knowledge permits, a generic revision of the Indian species. 



This revision has had interesting results from a morphological 

 point of view, for it shows that branchial structure and shell-form 

 are not necessarily correlated. In our new genus I ndoplnnorbis , 

 which has a typical discoidal shell, the branchial process is folded, 

 while both in the other Indian genera with shells of the same type 

 and in Camptoceras, the shell of which is more or less Physa-like, 

 the process is simple as in Planorbis, s.s. The contribution we 

 are now able to make to the anatomy of Benson's peculiar genus 

 leaves no doubt that Walker ' was right in assigning it to the 

 Planorbidae and that its resemblances to the Ancylidae, on which 

 we previously * laid stress, are hardly more than superficial. 



/. Shell flat and discoidal (Planorbiin.i.-)- Anal siplion 

 incomplete. 

 A. Shell rtlatively lar^je and thick with the whorls con- 

 vex above and below, both animal and shell clearly 

 sinistral. Penis without a horny stvlet. 

 i. Young- shell not as a rule Physa-\\\ie, animal with 

 a simple branchial process. Radula long and 

 narrov,', the cusps of the teeth thin and set on a 

 lower level than their bases. Penis short and 

 swollen when retracted, asymmetrical ; penis 

 sheath, with a thick-walled preputium, two re- 

 tractor muscles present ... ... ... Planorbis. 



ii. Young shell Pliysa-\\\ie. Branchi.al process lobed. 

 Radula relatively large and broad, with the 

 tips of the cusps of the teeth no thinner than the 



' Occ. pap. Mils. Zool. Univ. Michigan, .No. 64 fiQlQ 

 ' Journ. As. Soc. Bengal XIV, p. 457 (n}iQ). 



