1922. 1 N. Annandalr : Indian Planorbidae. 359 



has recognized four distinct types of male organ. All but one of 

 these are found in the Indian species, as well as a fifth. Slightly 

 modifying Simroth's definitions and adding one of the fifth type, 

 they may be defined as follows: — 



Type I. Penis short, bulbous, asymmetrical, without a peni- 

 al stylet, with an elongate, thick- walled praeputium. Sheath with 

 two retractor muscles. (Indian genera, Ivtha, gen. nov. ; ? Pla- 

 norbis Geoffroy.) 



Type II. Penis slender, elongate, asymmetrical at the tip, 

 with a comparatively short, thick-walled praeputium, without a 

 penial stylet. Sheath with a pair of ear-like processes above, with 

 a single retractor muscle. (Indian genus, Segmentina Flemming.) 



Type III. Penis cylindrical. S3'mmetrical, without a peninl 

 stylet, with a short, thin-walled but well differentiated praeputium 

 and two retractor muscles. (No Indian genus.) 



Type IV. Penis cylindrical, but asymmetrical at the tip, 

 with a horny stylet and a praeputium of complex structure. Sheath 

 with a single retractor muscle. (Indian genera, Gyraulus Agassiz ; 

 ? Diplodiscus Westerlund.) 



Type V. As in type III, but without differentiated praepu- 

 tium and with the penis ver5' long and sometimes coiled in the 

 sheath (Indian genera, tndoplanorbis Annandale and Prashad ; 

 Hippeutis Agassiz. ) 



Type V is closely allied to type III but may for convenience 

 be considered distinct. 



The radulae of the different genera are not so distinct as the 

 genitalia, but afford good characters in some instances. l\\ tndo- 

 planorbis the teeth are relatively large and the whole organ is 

 broad. In Hippeutis the lateral teeth are arranged in pairs. 



The jaws differ more markedly. In Indoplanorbis and 

 Gyraulus they consist of a comparatively stout but almost linear 

 transverse upper piece and of a pair of slender vertical side-pieces, 

 which in Indoplanorbis, as my assistant Mr. Sri Navasa Rao has 

 observed, are fragmented ; but in Segmentina (at any rate in some 

 species) and in Iniha the three pieces are completely broken up 

 into numerous horny teeth as in the Ancylidae. 



Key to the Indian Genera of Planorbinae. 



I. Shell comparatively thick nnd large (as a rule well over 

 i cm. in diameter) ; whorls convex above and below 

 without peripheral keel. No penial stylet. 



A. Young shell cliscoidal. Pseudobranch simple. 



Male organ of type I ... ... Planorbis. 



B. Young shell Physa-Uke. Pseudobranch when 



expanded ribbon-like with alternate depres- 

 sions and projections, lobed in contraction. 

 Male organ of type Y ...• ... Indoplanorbis. 



II. Shell small and thin, usually less than i cm. in diameter, 

 at least one surface more or less flattened 



.-I. Shell with internal ridges of enamel-like sub- 

 stance, of a flattened and truncate conoidal 

 shape. Male organ of tvpe II ... Segmentina. 



