184 Records of the Indian Museum. [Vol. XIV, 



In both the species only the middle reii;ion of the outer gills is special- 

 ized to serve as a marsupium, while a larger posterior and a much 

 smaller anterior portion retain the ordinary respiratory character. The 

 marsupial region in both species is much swollen and in the preserved 

 specimens is of a creamy colour. It is clearly marked off from the an- 

 terior and posterior respiratory regions. In P. micropleroides seventeen 

 water-tubes are modified to serve as a brood-pouch, while in P. ferru- 

 gineus only eleven are thus modified. 



The characters of the marsupium of the genus may be summed up 

 as follows. Marsupium formed by 11-17 simple water-tubes in the middle 

 region of the outer gills, leaving a larger posterior and a much smaller 

 anterior unmodified portion for respiration. 



From these characters it will be seen that Physunio should be included 

 in the group Exobranchiae of Simpson, and comes into his sub-group 

 Mesogenae. He established this sub-group to include the genera Cypro- 

 ijcnia and Ohliquaria, in both of which a variable number (7-23) of tubes 

 in the middle region of the outer gills are specialized as the marsupium, 

 a larger anterior and a shorter posterior region remaining respiratory. 

 The marsupium of Physunio agrees with the above two genera in having 

 the middle region of the outer gills modified, but differs in that the 

 shorter respiratory portion of the gills is anterior and not posterior as in 

 the American forms ; also the modified water-tubes are in no way speci- 

 ally elongated. Ortmann (3) as a result of his anatomical researches has 

 found it necessary to combine Simpson's sub-groups Heterogenae, Meso- 

 genae, Ptychogenae and presumably also the Eschatigenae into one 

 group. In these forms various portions of the outer gills are modified 

 as brood-pouches. Ortmann has included in the united group recog- 

 nised without special name all the forms in which a differentiated portion 

 only of the outer gills functions as the marsupium. 



The genus Physunio must, therefore, also be included here and not 

 amongst the Endobranchiae. The water-tubes (fig. 2) are filled with 

 glochidia throughout their length, and in the fully charged marsupium 

 the great antero-posterior distention of the water-tubes is very well 

 marked ; the inter-lamellar junctions also are much elongated in this 

 region. The glochidia were seen to be rather loosely attached to one 

 another by their larval membranes and could easily be shaken apart. 



Glochidiuin. — The glochidia of the two species dift'er mainly in size. 

 In the case of P. micro pteroides (fig. 3) a glochidium measures 0-32 mm. 

 by 0-29 mm., while that of P. ferrugineus (tig. 10) is much smaller, being 

 0-28 mm. by 0-26 mm. The glochidium of P. micropteroides is described 

 here in detail. 



The glocidium is of the bookless types. It has the usual two shell 

 valves. As seen in lateral view the shell is semi-circular with a curved 

 lower margin and an upper hinge-line. This line is nearly straight in 

 P. ferrugineus, a little curved in P. micropteroides. The shell is thin in 

 the middle and all over except along the hinge-line and the border, where 

 it shows a distinct thickening. The whole surface, except the border, ia 

 finely granulated. From the inner edge of the ventral border (figs. 4 and 

 5) a continuous flange projects inwards ; seen from the anterior or 

 posterior side this flange appears as a pointed tooth, but it is a con- 



