I92I.] 



Manipiir Molluscs. 



579 



als is gradual. The deiiticulations of the teeth are not separated 

 by any sharp Hne of division from the free lobe, but the whole 

 structure is continuous as in Limnaea. 



The genital aperture, which is situated some distance behind 

 the left tentacle, is slit-like and relatively large. The vas deferens 

 opens directly without in- 

 vagination into an elongate 

 cylindrical eversible penis 

 with a bulky lumen. This 

 organ is slender, tubular 

 and of considerable length, 

 extending practically to the 

 end of the penis- sheath 

 when contracted. Its ori- 

 fice is circular and symme- 

 trical. It is similar in form 

 to the penis-sheath, which 

 is a long, somewhat curved, 

 thin-walled tube. There is 

 no retractor muscle. The 

 whole apparatus is, there- 

 fore, much less complicated 

 than any of the types recog- 

 nized by Simroth. There is 

 no penial stylet. Tlie female 

 genitalia are of the usual 

 type. 



Type-species. — Planor- 

 bis exusttis, DesLayes. 



This genus differs from 

 the true Plauorbis of Miiller 

 (with the European P. cor- 

 neus, Linn., as type-species) 



in the structure of the branchial process, the radula and the 

 genitalia. The shell also has a characteristic facies, although it 

 is difficult to formulate the differences precisely. The shape of 

 the aperture is noteworthy. The branchial process apparently 

 resembles that of Bullinus ' and Miratesta,' while the radular 

 teeth are not unlike those of the subgenus Galba ' of the genus 

 Limnaea. 



As Mons. L. Germain shows in his catalogue of the Planor- 

 bidae in the collection of the Indian Museum (now in the press) 

 all of the larger discoidal shells of the family from the Indian 

 Empire that are preserved in the collection belong to Planorbis 

 exuzttis, Desh., the type-species of our new genus A consider- 



_.ep 



u 



B. 



Fig. 14. — A. Penis and end. of vas deferens 

 of I)idoplanoi-bis exitstus (Deshayes). 

 e.p. external opening; /i. penis. ; p.s. 

 penis sheath ; sp. sperm duct ; v d. 

 vas deferens. 

 B. Terminal part of the male duct of 

 /. exustiis. 



1 Pelsenecr, Mollitsca, in Lankester's Treatise on Zoology, p. 186. 

 (I9''6). 



■2 P. and K. Sarasin, Shss'm. Moll. Celebes, pp. 72-77, figs. 165, i66. 

 5 Baker, Chicago Acad. Sci. Spec. Pub. No. 3, p. 199 ('1911). 



fig. 



175 



