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



marginals differ from those of G. convexiusculus described in a pre- 

 vious paper ' in having onlv three or at the most four cusps. 



The genitalia resemble those of G. convexiusctdus {loc. cit.) in 

 all essentials. The stylet, however, is better developed and near the 

 tip has the margin on one side produced into a distinct triangular 

 flap. This is one of the common .species of Planorbidae in the 

 Manipur Valley and was collected in large numbers in the Loktak 

 Lake, and in the streams and swamps surro\niding the lake. 



Hippeutis, Agassiz. 



1837. Hippeutis, Agassiz, op., cit., p. 22. 

 1886. Hippeutis, Clessin, op. cit., p. 34. 



The shell of Planorhis umhilicalis, Benson, as Bavay and 

 Dautzenberg* have observed, agrees in generic type with that of 

 the European species of Hippeutis, of which Planorhis fontanus, 

 Lightfoot, is the type.^ We have been unable to find any account 

 of the anatomy of this mollusc and cannot therefore be certain as 

 to the generic identity of the form here discussed. In both radula 

 and soft parts it differs very widely from the Indian species of 

 Segmentina and Gyraulus. which agree, at any rate in the genitalia, 

 with S. nitidtis and G. albns, respectively the type-species of the 

 two genera. 



In shell-characters P/aworSis caenosus, Benson, closely resembles 

 H. umbilicalis , and the two species are probably- congeneric, as 

 also appears to be the case with Planorhis sindicus of the same 

 author. In the account of the Fauna of the Inle Lake* an unfor- 

 tunate confusion between caenosus and trochoidcus has occurred. 

 The latter is certainly a Segmentina, while the shell of the former 

 has no internal ridges and, as stated above, is probably a Hippeu- 

 tis. 



Hippeutis (?) umbilicalis (Benson). 



1857. Planorhis umbilicalis. Benson, Journ. As. Soc. Bengal V, p. 471. 

 1876. Planorhis umbilicalis, Hanley and Theobald, Conch. Indica, p. 



xviii, p. 18, pi. xl, fius. 7-9 

 1878. Planorhis (Segmentina \ umhilicalis, Nevill, op. cit., p. 246. 

 18S6. Planorhis umhilicalis, Clessin, op. cit., p. 136, pi. .\v, fig. 6. 

 1910. Planorhis {Hippeutis) umhilicalis, Bavay and Dautzenberg, 



Journ. CoHcliyltol. LVllI, pp. 18, 19. 

 1915. Planorhis (Segmentina) umbilicalis, Preston, op. cit., p. 125. 



No septa are to be seen in the large number of both young 

 and very large specimens of this species in the collection. The 

 shell in other respects closely agrees with Benson's original descrip- 

 tion and the excellent figures of the species in Hanley and Theo- 

 bald's work. 



The external soft parts generally resemble those of Gyraulus 



I Pec Ind Mas- XVlIl, p. 54, fig. SB (1919). 



■^ Journ. Concliyliol. LVIII, pp. 18, 19 (iqio). 



3 See also Journ. Linn. Soc. London XXXI V, p. 213 (1920). 



* Pec. Ind. il/iis. XIV, p. 1 13 ; also in yVt-c. Geo. Stirv. Ind. L, p. 219. 



