18 DESCRIPTIONS OF 



apex, there are thi'ee or four gi'anules in a line. There are live specimens before me ; 

 two adult, both of which I suspect are females, and three about half grown. The 

 general form and emargination of the base, give it very much the appearance of a 

 horse bean, whence the name. 



UnIO SAGITTARIUS. PI. 26, fig. 12. 



Testa laevi, vald6 transversa, vald6 inaequilaterali, subinflata, suboylindracea, postice acuto-angulata ; val- 

 vulis subtenuibus ; natibus vix prominentibus ; epidermide virido-olivacea, striata ; dentibus cardinali- 

 bus longis, rectis lamellatisque ; lateralibus prislongis subrectisque ; margarita alba et iridescente. 

 Shell smooth, very transverse, very inequilateral, rather inflated, rather cylindrical, 

 acutely angular behind ; valves rather thin ; beaks scarcely prominent ; epidermis 

 greenish olive, striate ; cardinal teeth long, straight and lamellar ; lateral teeth very 

 long and nearly straight ; nacre white and iridescent. 

 Proc. Acad. Nat. Sci. Vol. 8, p. 93. 



Eah. — Siam. S. R. House, M. D. 



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

 Diam. -6, Length -8, Breadth 2 inches. 



Shell smooth, very transverse, very inequilateral, rather inflated, rather cylindrical, 

 acutely angular behind ; substance of the shell rather thin ; beaks scarcely prominent, 

 nearly terminal : ligament very long and thin ; epidermis olive green, darker on the 

 posterior portion, striate and without rays ; umbonial slope subangular ; cardinal 

 teeth long, straight, lamellar, double in the right and single in the left valve ; lateral 

 teeth very long, nearly straight, single in the right and double in the left valve ; anterior 

 cicatrices distinct ; posterior cicatrices confluent ; dorsal cicatrices placed in the centre 

 of the cavity of the beaks ; cavity of the shell rather deep and rounded ; cavity of the 

 beaks scarcely perceptible ; nacre white and iridescent. 



Remarks. — There were two specimens and an odd valve only of this species among 

 the Siamese shells brought by Dr. House. In general form and characteristics, it is 

 exceedingly like U. Gailliaudii, Fer., from Egypt, and might at first sight be taken for 

 that species. It is, however, entirely different in the cardinal teeth as to form, size 

 and condition. Our shell has lamellar teeth, double in the right and single in the 

 left valve, while the Caillmudii has short, thick teeth, double in both, valves. Both 

 my specimens of Cailliaudii have minute rays over the whole disk, while sagiUarius 

 seems to be entirely without them. It must be remarked, however, that the speci- 

 mens are by no means perfect. Neither of them have the beaks perfect enough to 

 ascertain if they have been plicate. In outline this species has also some affinity to 

 U. Ingalkianvs, (nobis,) but it is much more transverse and more cylindrical. 



