NEW FRESH WATER AND LAND SHELLS. 17 



fokls. In the crispatus the folds cover the whole surface. In our shell they are 

 smaller and extend over the upper part only of the valve, and down the posterior 

 slope. In the teeth they are quite different ; those of the crispatus being thicker and 

 the cardinal teeth being shorter and double in both valves. The folds are small and 

 numerous, and may be seen on the inside of the valves in the region of the beaks and 

 the posterior slope. The size of humilis is about that of U. acutissimus, (nobis,) but is 

 not so angular on the posterior margin, and the folds are smaller and more numerous. 

 It differs also in color, the acutissimus being more yellow. 



Unio piiaselus. pi. 26, fig. II. 



Testa plicata, transversa, inflata, subemarginata, vald6 insequilaterali, posticc obtuse angulata ; valvulis 

 crassis ; natibus parvis, prominulis, ad apieem undulatus ; epidermide viridi et lutca, polita ; dentibus 

 cardinalibus brevis subcrassisque ; lateralibus prBelongis, in uteroque valvulo duplicibus subcurvisque ; 

 niargarita alba et iridesconte. 

 Shell plicate, transverse, inflated, subemarginate, very inequilateral, obtusely angu- 

 lar behind ; valves thick ; beaks small and somewhat prominent, folded on the apex ; 

 epidermis green and yellow, polished ; cardinal teeth short and somewhat thick ; 

 lateral teeth very long, double in both valves and somewhat curved ; nacre white and 

 iridescent. 



Proe. Acad. Nat. Sci. Vol. S,' p. 94. 



Hob. — Siam. S. R. House, M. D. 



My cabinet and cabinet of W. A. Haines, New York. 

 Diam. -5, Length -5, Breadth 1-1 inch. 



Shell folded, wide, inflated, subemarginate at base, very inequilateral, obtusely 

 angular behind ; substance of the shell thick ; beaks small, a little prominent, folded 

 on the apex ; folds small, few, green, extending over the superior portion and 

 on the posterior slope ; ligament rather short and thin ; epidermis rather a light yel- 

 low, highly polished, with a few transverse green lines, some of which are disposed to 

 take a zigzag form ; umbonial slope rounded ; cardinal teeth short, rather thick and 

 disposed to be double in both valves ; lateral teeth verj- long, somewhat curved and 

 double in both valves ; anterior cicatrices distinct ; posterior cicatrices confluent ; dorsal 

 cicatrices placed on the under posterior part of the cardinal tooth ; cavity of the shell 

 deep ; cavity of the beak rather shallow and rounded ; nacre white and very iridescent. 



Remarlcs. — This species belongs to a group in which may be included gratiosus, 

 Phili., crispatus, Gould, humilis and nucleus herein described. In outline it very much 

 assHuilates to crispatus, but it diflers very much in having few folds, in being highly 

 polished, less angular at the posterior margin, and more rounded and inflated on the 

 posterior slope. The younger specimens have a strong resemblance to humilis, but 

 are more polished, have fewer fokls and are thicker. On the umbonial slope near the 



