NEW FRKSII AVATEU AND LANI> SlIKLLS. 37 



The above described species is more like Anodon radiaius, Spix, (Test. Fiuv. Bras, 

 pi. 23, fig. 1,) than any other South American species. It is about the same size, 

 but in outline it differs by being more oblong and less elliptical. It differs also in 

 not being rayed. It is very nearly of the same outline vi^ith Anodonta fluviatUis of 

 our rivers, but it cannot be confounded with that species, as it has the peculiarly formed 

 fosset at the posterior end of the ligament, which is so remarkable in the South 

 American AnodontcB. The singular crimpled rays in the epidermis are very remarkable, 

 but it may not be a permanent character of the species. It is to be regretted that a 

 single specimen only has come under my view. I owe this one to the kindness of the 

 Rev. Mr. Holton, who mentions in his travels that they were in abundance near 

 Cartago, and to him I dedicate it. 



Family NAIADES. 



Subgenus PLAGIODON.* 



Testa zequivalvis, ina3quilatcra, oblique trigona, valdo inflata ; dentibus cavdinalibus crcnulatis, coniprcs.sis, 

 transversis, curtis, in utroque valvule duplicibus ; dentibus latcralibus nuUius ; ligamcntum vix cx- 

 trorsuiu ; impressio muscularis antice composita. 

 Proc. Acad. Nat. Sci. Vol. 8, p. 79. 



Shell equivalve, inequilateral, obliquely trigonal, very much inflated; cardinal 

 teeth crenulate, compressed, transverse, short, double in each valve ; no lateral teeth ; 

 muscular impression composite before. 



This subgenus is proposed for a fluviatile shell, which is nearer to Monocondyloea, 

 D'Orbigny, than to Margaritana, Schumacher, and may be considered properly, as so 

 interposed as a link to connect them. It has the epidermis of Monocondyloea, being 

 of a dull cloth-like texture, and not polished and smooth, like the Unio generally is. 

 The point in which it chiefly differs is the peculiar structure of the teeth. As in 

 Monocondyloea there is no lateral tooth, the part of the dorsal margin where that 

 exists in the Unio forming a i^late, is here simply a line, forming a margin over a deep 

 cavity, on which line is placed a thin delicate ligament. The cardinal tooth is large, 

 but unlike the Margaritana it is transversely compressed, and the upper division is 

 longer and begins immediately over the lower one, between it and the beak. In 

 Monocondylcca the cardinal teeth consist of one smooth rather large lobe or callus, in 

 each valve, passing each other, and fitting into a corresponding depression in the 

 opposed valve. If we may judge from a single specimen of Plagiodon I should 

 conclude that the substance of the valves was never so thick as that of Monocondylcca. 

 It is much more inflated, the three diameters being nearly the same, that through 

 the axis being rather smaller. Being a very light shell, and having so peculiar a 

 form, it cannot be very locomotive. From its rotundity, and having the beaks nearly 



' «,aj/;r. tiiiiitVcrsH; ami dst. (it'll;. 

 10 



