306 



THE PEARLY FKESR-WATER MUSSELS— SIMPSON. vol xviii. 



COMPARISON OF DESCRIPTIONS OF UNIO PRESSUS AND MARGARITANA RUGOSA. 



Unto 2>^'esstts, Lea. 



Branchiae large, rounded below, free 

 nearly the whole length of the abdom- 

 inal sac. 



Palpi small, subangular, united half- 

 way down the posterior edges. 



Mantle thin, slightly thickened on the 

 margins. 



Branchial opening large, blackish on 

 the edge, and with numerous papilhe. 



Anal opening rather small, blackish, 

 and without papillif . 



Superanal opening rather large, united 

 for some distance below, blackish on the 

 edges. 



Color of the mass, dirty white. 



Embryonic shell subtriangular, light 

 brown: with hooks. 



Margariiana rugosa, Barnes. 



Branchi;* very large, rounded below, 

 the inner ones much the larger, free the 

 whole length of the abdominal sac. 



Palpi rather small, subangular, united 

 nearly halfway down the posterior edges. 



Mantle rather thin, much thicker on 

 the margins, blackish on posterior, basal 

 edge. 



Branchial opening rather large, with 

 small brown papilhe. 



Anal oi)ening rather large, without 

 papilhe. 



Superanal opening very large, with a 

 dark brown line within, united below. 



Color of the mass, salmon. 

 Embryonic shell triangular, brown; 

 with hooks. 



Ill Margaritana complanata, Barnes, which has a beak sculpture 

 quite like that of U)iio pressus, but coarser, a similar arrangement of 

 teeth is seen, though the shell is heavier, more rounded, and the hinge 

 plate is broader. In many specimens the hinge of the right valve is 

 completely cut away at the beaks, and the cavity is filled by a corre- 

 sponding tooth in the left valve. Unio charlottensis, Lea, from North 

 Carolina, an undoubted member of this group, has a form approach- 

 ing that of Margaritana complanata, but it is rather more elongated. 



Margaritana holstonia sometimes exhibits laterals, and in general 

 form, size and texture so closely resembles some of the si)ecies of the 

 group of Unio nashi-'illensis, that even Dr. Lea occasionally labeled them 

 wrongly. 71/. confragosa, Say, resembles no other Margaritana at all, 

 but approaches more nearly in form to the Unios of the Lachrymosus 

 group, and the animal is remarkably close to those of that assemblage. 

 Unio hiesianus, Hende, of China, has the same kind of blurred laterals 

 as the American Margaritanas, but it appears from conchological char- 

 acters to be a member of the group of Unios typified bj^ U. sinensis, 

 liCa. I have dwelt at length on this part of the subject because the 

 partial or total want of lateral teeth in the species of Margaritana is a 

 very curious feature. I can only believe that they are all true Unios 

 whose teeth have been modified or injured by conditions of water, food, 

 bottom, or some other element of environment. In some of them, where 

 the laterals have merely become obsolete, such as those of the Margarit- 

 i/era group, I think the explanation is easy. M. monodonta and Jiildreth- 

 iana are found in running water under stones, buried slightly in the mud, 

 and U. hembeli lives in the nearly stagnant bayous of Louisiana, so that 

 a strong, toothed hinge is not required to hold the valves in place. The 

 heavy-shelled species that live in running water have blurred laterals 

 which appear as if diseased, and it seems not improbable that they may 



