4o6 Records of the Indian Museum. [Vol. XVI, 



The inner lamellae of the inner pair of gills are attached to the 

 abdominal sac only along one-third of their anterior length, while 

 the posterior two-thirds is quite free until the lamellae of the oppo- 

 site sides meet each other a little behind the posterior margin of 

 the abdominal sac and unite to form the diaphragm. The outer 

 lamellae of the outer pair of gills are united with the mantle all 

 along. The diaphragm is complete and is formed only by the gills, 

 there being no mantle connection between the branchial and the 

 anal apertures, though the mantle-wall of the opposite sides is 

 slightly drawn in. The palpi are well developed, rather ellipsoid 

 in outline, with a narrow base of attachment to the abdominal 

 mass and further attached along one half of their posterior margin 

 to the mantle. The anterior margin of the gills is separated from 

 the palps by a small gap. The branchial aperture is large with 

 two to three rows of elongated papillae , the anal is a little more 



IG. 



Text-fig. 2. — Animal of M.voiidemhiischiana, var. c/i a peri. Aii.=:ana\ 

 aperture; 5r.= branchial aperture ; F.=foot ; /.G.= inner gill; O.G. outer gill ; 

 P, palp ; Sa- supra-anal. 



than two-thirds the size of the branchial and is quite smooth ; 

 the supra-anal is much smaller than either and is separated from 

 the anal by a small mantle connection. The foot is fairly large. 



A comparison of the above description of a form of the genus 

 Monodontina with that of Pseudodon described in my former paper 

 {loc. cit., p. 295) shows that the two genera differ from each other 

 in the following respects : — 



1. The inner lamellae of the inner pair of gills of Monodontina 

 are free from the abdominal sac along two- thirds of their length, 

 while in Pseudodon they are attached all along. 



2. There is a distinct supra-anal in Monodontina, but owing 

 to the absence of a mantle connection in the position between 

 the anal and supra-anal of other genera there is no distinction be- 

 tween the anal and the supra-anal in Pseudodon. 



3. The anal is quite smooth in Monodontina, whereas its lower 

 part is papillose in Pseudodon. 



