294 



Records of the Indian Museum. 



[Vor.. XVI, 



Physunio, Simpson. 



Ghosh {loc. cii., p. 112) has described the gills of the animals 

 of two species of this genus, but the structures are variable. 



The animal (fig. 5) may be described as follows : — The gills are 

 much broader in the anterior than the posterior half and the outer 

 pair is much smaller in breadth than the inner, so that the latter 

 projects below it all along. The attachments of the gills are similar 

 to those in Parreyssia and Lamellidens, except that the portion of 

 the inner lamellae of the inner pair of gills, which is attached to the 

 abdominal sac, is much less than half their length. Only a portion 

 of the outer pair of gills is marsupial, a small anterior and a much 

 larger posterior portion of them remaining unmodified for res- 

 piratory purposes ; the marsupial part is formed by 11-17 simple 

 water-tubes. The free margin of the marsupial part of the gills 

 in this genus also remains sharp.' The outlines of the placentulae 



rp:xT-FiG. 5. — Animal nl P/iysiinio ferrngineus, reference lettering same as 

 in fig. I. 



cannot be definitely described, as the glochidial membranes are 

 very loosely attached to one another and a compact structure is 

 not formed. The palpi are triangular with the apex rounded and 

 attached by a broad base. The margin of the mantle is feebly 

 thickened and is entire. The foot is comparatively^ small in pre- 

 served specimens, though it is an important organ for burrowing 

 in the mud.^ Dr. Annandale tells me that it is capable in life of 

 considerable expansion. The branchial aperture is about one and 

 a half times as large as the anal, and its margin has three rows 

 of elongated tubercles. The mantle ridge separating the branchial 

 from the anal aperture is well developed, but small, owing to the 

 diaphragm extending to very near the margin. The mantle con- 



' Detailed description of the marsupium, etc., is given in my paper I?ec. /mi. 

 Mus., XIV, pp. 183-1S5 (1918). 



2 See Annandale, Rec. Ind. A/its.. Xl\', p. 141 1 1918). 



