1895. 



PROCEEDINGS OF THE NATIONAL MUSEUM. 313 



which, wlien young, has usually well-developed caidiuals. I believe 

 both should be placed iu the geuus Unio. Graspedodonta (Kuster, MS.) 

 diftersfrom .4?«>rfo/?/a by a peculiar thin lamella at the liinge of the 

 left valve, and is founded on Anodonta smanKjdina, Anton. ' 



Th& locality given is uncertain, but Clessiu thinks it may be America. 

 The figure represents what is probably an immature shell, of a species 

 unknown to me, and is, I think, a young Cristaria with a rather high 

 dorsal ridge, which may be C. herculea, Middendorf. 



Genus ARCONAIA, Conrad. - 



In the rivers of China and southeastern Asia certain peculiarities of 

 environment seem to exist, which, in some cases, wonderfully modify 

 the teeth of bivalves, and in others produce curious distortion. Men- 

 tion has been made in this paper of the remarkable vertical striation 

 of the teeth of some of the Chinese Unios, a character which is not 

 confined to this region, and may be a mechanical development to 

 strengthen the shell. A large number of these Unios are strangely 

 distorted, of which an account will be given later under the heading 

 "Oriental Eegion," in the discussion of geographical distribution. 



In the Arconaias the twisting is both axial and lateral, and I have 

 no means of knowing whether or not this contortion is always in the 

 one direction. However, in certain species of Unios in the groups 

 typified by U. pisciculus, Heude, and U. triformis, Heude, the shells 

 may be turned sharply at the posterior end either to the right or the left. 

 It is doubtful whether Arconaia is generically distinct from Unio, but 

 as the anterior part of the shell is always developed into a little wing, 

 and the cardinals difier somewhat from those of any Unio I know of, 

 it is perhaps best to let it stand as a genus. According to Deshayes,^ 

 A. contorta has the mantle lobes separable as in Unio. 



Genus PSEUDODON, Gould.' 

 The species which are now generally included under this generic 

 name were placed by Lea and other authors in Monocondylcca—-dn 

 unrelated South American genus— on account of the similarity of the 

 hinge characters. In most of them a single rounded cardinal tooth or 

 tubercle is found in each valve, and there are no laterals present. 

 Lequminaia, Conrad, consisting of a few species of compressed iS^aiades 

 from southern Europe and western Asia, with vestiges of cardinals, 

 which genus was included by Dr. Lea in Monocoiidylcea, is now generally 

 regarded as a valid genus, so that all the species I should place in 

 Psendodon are confined to southern and eastern Asia, and a few of the 

 islands of the Malay Archipelago. _ 



The group, even when separated from Monocomhjkm and Legiiminam, 



MClessiu, in Mart. Chem. Conch. Cab. (Anodonta), p. 93, 1876. 



2Amer. Jonrn. Conchology, I, p. 23i, 1865. 



3 Jouru. de Conch., XXII, p. 85. 



^Proc. Bost. Soc. Nat. Hist., p. 161, 1844. 



