330 THE PEARLY FRESH-WATER MUSSELS— SIMPSON. vol. xviii. 



Thibetan plateau, including, probably, the Indus on the west and the 

 Hoang-Ho on the northeast, is inhabited by a peculiar Unioue fauna. 

 With this region must be included Japan, Ivorea, Manchooria, Formosa, 

 the Philippines, and probably all the islands of the Malay Archipelago, 

 which are peopled with Naiad life, to and including the Solomon group. 

 The genus Unio is everywhere abundant throughout this area, and 

 Pseudodon is common to nearly all of it. A magnificent set of Ano- 

 dontas is developed in northern China, and in this region Cristaria, 

 Lepidodesma and Arconaia are found. Solenaia inhabits the greater 

 part of the area. 



Dr. Lea was led to believe that two or three of the Unios of the 

 southern part of this region were found in Australia, but later he was 

 convinced that this was an error, and that no species of the two fam- 

 ilies is common to the Oriental and Australia]) regions. The Xaiad 

 fauna of this region is magnificent and diversified, and almost rivals 

 that of the Mississippi Valley in vigor, size, solidity and variety of forms. 

 Both Dr. C. A. White and von Ihering believe that the Unios and 

 Anodontas of this area are closely related to those of the central part 

 of North America. Not only does there seem to be a general relatiou- 

 shij) among a large number of the Naiads of this province with those 

 of the Mississippi basin, but several Oriental groups are apparently so 

 close to those of our own region that it is well-nigh impossible to sei)a- 

 rate them. Thus, the Asiatic Anodontas typified by A. icoodiana. Lea, 

 il found in the United States, would be placed by most students with 

 A. plana; the Chinese Unios of the group of U. housei, Lea, and myers- 

 ianus, Lea, are evidently quite near the Alatus assemblage j Unio 

 SKperhus, Lea, is very much like our f7. capax, Green, and a number of 

 the tuberculate forms of China could almost be placed in the American 

 groups of U. lachrymosus and U. pustulosus. 



Certain peculiarities of shell growth are remarkable among the Naiades 

 of this entire region. One of these is the loss or partial degeneration 

 of the hinge teeth, and another is the remarkable development of ver- 

 tical tooth striation, to both of which attention has already been called 

 in this paper. The third is the singular contortion of many of the spe- 

 cies, of which there are three varieties. The first and simplest is a mere 

 bending of the posterior part of the shell, either to the left or right, 

 something like that of a Tellma^ which is seen in two or three groups of 

 elongated Chinese species. Some of these forms are bent into a strong 

 curve. The second is a twisting of the shell on its axis, which occurs 

 in the Arconaias and some of the Unios.' These two forms of distortion 

 may occur in the same species. The third and most strange form of 

 irregular growth is seen in a number of very solid, oval and somewhat 



^ Arconaia provancheriana, Pilsbrr, which is twisted on its axis like a Parallelo- 

 pipedon, is no doubt a distorted form of Vnio complanatus, Solander, from Canada, 

 and does not come from China, as has been surmised. (See Naturaliste Canadien, 

 XIX, p. 171, 1889.) 



