igig.J B. Prashad : Astatic Unionidae. 409 



to him this subgenus differs from Unio, s.s. in both shell charac- 

 ters and the soft parts. He stated in his paper that the account 

 of the subgenus would be published later, but so far no such 

 account has appeared. 



In Dr. Annandale's collection there are three specimens of 

 Unio (Rhombunio) semirugatus preserved in spirit ; these were 

 identified as Unio simonis by Preston. One of the three specimens 

 is a gravid female. The following description of the soft parts of 

 the sub-genus is based on this material. 



Corresponding to the shape of the shell the gills (fig. 3) are fairly- 

 broad but rather short ; the inner pair being much broader than 

 the outer, particularly in the anterior half. There is only a very 

 small gap between the anterior margin of the gills and the posterior 

 margin of the palpi. The outer lamellae of the outer pair of 

 the outer gills are attached to the mantle all along their length. 

 The inner lamellae of the inner pair of gills are free from the abdom- 



O.C. 



Tkxt-fig. 3. — Animal of Unio {RJiombiinio) semirugatus, reference letter- 

 ing same as in fig. 2. 



inal sac except for a very short distance near the extreme ante- 

 rior end ; posteriorly the lamellae of opposite sides unite to form 

 the diaphragm. The diaphragm is formed entirely by the gills, the 

 mantle taking no part in its formation. The outer pair of gills 

 alone are marsupial, but in these also a very small anterior and a 

 much smaller posterior portion of each is not modified for a marsu- 

 pial function. The margins of the marsupial gills are quite 

 sharp even when the gills are charged with glochidia. The water- 

 tubes in the gills are simple but well developed. The septa in the 

 outer pair of gills are very crowded in the female, but in the male 

 the arrangement is practically the same as in the inner pair of gills. 

 The palpi are ellipsoid, attached to the abdominal sac along their 

 base, and along nearly half of their posterior margin to the mantle. 

 The branchial, anal and supra-anal openings are as is usual ; the 

 mantle connection between the anal and the supra-anal being near- 

 ly equal in length to the supra-anal and slightly larger than the 

 anal, while the branchial is much larger. The branchial bears 



