1895. PROCEEDINGS OF THE NATIONAL MUSEUM. 305 



tbese edentulous forms have been generally called Anodontas. But it 

 often happens that in a lot of individuals of a single s[)ecies taken from 

 one locality, there will be found e\"ery variation from perfect teeth to 

 the merest vestiges. For this reason, and on account of the fact that 

 most of the shells of the group have beautiful, delicate, chevron-shaped 

 beak sculpture, which often extends well on to the body of the shell, of 

 a form quite characteristic of many Oriental Unios, I have no doubt, 

 althougli we know nothing of the soft parts of the members of this 

 group, that they must be placed with Unio. Some of the Margaritauas 

 evidently belong with well-known groups of Unios. In the group of U. 

 manjaritiferus, 1 have given examples. Margaritana rugosa^ Barnes, 

 sometimes approaches so closely in external appearance to UniopressuSy 

 Lea, that one is labeled with the name of the other by competent stu- 

 dents. It has the same compressed, elongate-rhomboid form, and both 

 are rayed alike; the only essential difference in appearance being that 

 the former is usually somewhat corrugated on the posterior slope, while 

 the latter is without sculpture. Immediately under the beak in the 

 right valve in either species, the hinge plate is almost or entirely cut 

 away. Just before this is a single cardinal, usually somewhat com- 

 pressed, and on the i^osterior part of the hinge plate is a more or less 

 perfectly developed lateral. It is usually considerably blurred, even in 

 the Unio. 



In the left valve there is a triangular, compressed tooth directly 

 opposite the missing portion of the hinge plate in the left valve, which 

 is generally curved backward, and tits into the gap almost perfectly. 

 Just before this is a slightly developed, compressed cardinal, and 

 behind it in the Unio two not very perfect, elongated laterals. In the 

 MaTgaritana the laterals of the left valve are generally blurred ; some- 

 times in old shells they are shown as a sort of rounded ridge, but 

 frequently they run more or less diagonally across the hinge plate and 

 point downward posteriorly, as they do in other species of the genus. 



In a specimen of Unio pressns before me, from White River, Indiana, 

 the same direction is taken by the laterals of the left valve ; the lower 

 one running out and ending at the ventral edge of the plate, attaining 

 only a little more than half the length of the other. In rare instances 

 the laterals of the Margaritana are nearly perfect, and those of the Unio 

 quite blurred. The sculpture of the beaks in both species is nuich 

 alike, that of the Unio being finer. In both, it has a tendency to fall in 

 two loops from points on either side of the beaks. The soft parts of 

 these species are singularly alike. 

 Proc. N. M. 95 20 



