206 NEW UNIONIDiE OF THE 



anion"- other species from the vicinity of the Hot Springs of Arkansas. It is a 

 very remarkable species, belonging that group of which irroratus (nobis) may be 

 considered the type, judging from the general form and the outline. But, unfortu- 

 nately, the epidermis on both valves is so much worn and deteriorated that the 

 character of the markings cannot be observed. Perfect specimens may present 

 minute greenish spots like irroratus, or the epidermis may be like pustulosis, (nobis.) 

 More than one-half of the disk is covered with coarse tubercles, the remaining 

 part, the umbonial and posterior slopes are covered with about ten rough, slightly 

 curved folds. The beaks are very much raised and pointed, but they are so much 

 eroded that the nature of the undulations cannot be observed. The most remark- 

 able characteristics of this species is the enormous size and thickness of the cardi- 

 nal teeth. From the point of the beaks perpendicularly to the end of this tooth 

 in one specimen, the mass is one inch and four-tenths, and transversely it is one 

 inch and five-tenths, while the valve itself is but three and seven-tenths by two 

 and seven-tenths inches. Wood, in his " General Conchology," pi. 22, figs. 1 — 4, and 

 in his " Index Testa." pi. 2, fig. 29, describes and figures a specimen, {Mya 

 nodulosa,) in outline and tubercles very closely like this, but the lateral tooth 

 has regular, equal, parallel strife, which places it in Prisoclon, Schum. = Castalia, 

 Lam. The description is so short and imperfect as really to be useless, and the 

 habitat is unknown. 



In the Imperial Cabinet in Vienna, I saw, in 1853, opposed valves of two indi- 

 viduals of "Wood's shell, the cardinal and lateral teeth of which were both regularly 

 striate. It was there named Ghama plumbea, Mlillfeld. Prof. Fraunfeld, the able 

 zoologist of the Museum, informed me that it had long been in the cabinet and 

 its habitat was entirely unknown. (See my notice, Proc. Acad. Nat. Sci., vol. vi. 

 p. 368.) In the MS. for my next edition of Synopsis, I have transferred Wood's 

 nodulosa to the genus Prisodon. By the liberal permission of the Secretary of the 

 Smithsonian Institution, one valve will remain in my collection. 



Unio Arkansasensis. PI. 30, fig. 275. 



Tests, lsevi, ovato-obliqua, insequilaterali, postice compressa et obtuse biangulata, antice rotunda; 

 valvulis crassiusculis, antice paulisper crassioribus; natibus subelevatis; epidennide flaveseente, 

 obsolete radiata ; dentibus cardinalibus parvis, striatis crenulatisque ; lateralibus sublongis, subrectis 

 subcrassisque ; margarita alba- et valde iridescente. 



Shell smooth, ovately oblique, compressed behind, inequilateral, obtusely biangular 

 behind and rounded before ; valves somewhat thick, slightly thicker before ; beaks 

 somewhat raised ; epidermis yellowish, obscurely radiated ; cardinal teeth small, 

 striate and crenulate ; lateral teeth rather long, nearly straight and thick ; nacre 

 white and very iridescent. 



Proc. Acad. Nat. Sci., 1862, p. 169. 



