CHIEFLY OF THE UNITED STATES. 287 



Schum. = CastaUa, Lam, These strise are not so strong and well developed as in 

 Prisodon amhigtm, Solium., but they are moderately developed and, probably, entirely 

 normal to the species. The apex of the beaks is not perfect in this species, and whether 

 it has undulations or not cannot be ascertained from this specimen, but the oblique 

 undulations on the posterior slope are probably well developed to the points. The 

 epidermis is smooth and glossy, and of so dark an olive as to be nearly bottle-green. 

 In the outline and general appearance one is reminded of Unio ellipsis (nobis). Be- 

 fore Triquetra contorta, from China, was described by me, none of us could have ex- 

 pected to see a member of the Unionidce to be curved like Area tortuasa, Lin. ; but 

 now we have a second member of the family totally unlike the other, except having 

 a curved plane of the shell. Since the above was written I have had two specimens 

 from Hong Kong referred to me by Mr. J. G. Anthony. They have both the same 

 inequivalve character, but they differ in having tubercles on the disks as well as in 

 having folds. The strije of the lateral teeth are much stronger, thus presenting the 

 character of Prisodon. This cliaracter, with the tubercles, suggests that this species 

 may be identical with Wood's Mi/a nodulosa, which I have placed in the genus 

 P)~isodon. 



Note. — Since the above description was published in the Pro. Acad. Nat. Sci., April 18th, 1865, 1 have seen 

 the description of a species by Messrs. Crosse and Debeaux, under the name of U. Tientsinensis, in the 

 Journal de Conch., July, 1863. (See note page 285.) 



Anodonta Youkanensis. pi, 40, fig. 99. 



Testa Isevi, elliptiea, valde inflata, valde insequilaterali, iDostice subbiangulata, antice rotundata ; 

 valvulis subcrassis ; natibus prominulis, ad apices aliquanto undulatis; epidermide tenebroso-fusca, 

 eradiata ; margarita albida. 



Shell smooth, elliptical, very much inflated, very inequilateral, somewhat bian- 

 gular behind, rounded before; valves somewhat thick; beaks a little prominent, 

 somewhat undulate at the tips ; epidermis dark brown, without rays ; nacre whitish. 

 Proc. Acad. Nat. Sci. 1867, p. 81. 



Hah. — Upper Youkan River, Arctic America. 



My cabinet and cabinet of Smithsonian Institution. 

 Diam. 1-7, Length 2-5, Breadth 4-9 inches. 



Shell smooth, elliptical, very much inflated, very inequilateral, somewhat bian- 

 gular behind and rounded before ; substance of the shell somewhat thick, rather 

 thicker before; beaks a little prominent, somewhat undulate at the tips; ligament 

 very long and narrow ; epidermis dark brown, without rays and with very close 

 marks of growth ; umbonial slope flattish ; posterior slope long ovate and slightly 

 carinate ; anterior cicatrices couihient, large and well impressed ; posterior cicatrices 



