38 DESCRIPTIONS UF 



terminal, incurved and recurved, it at once reminds one of Isocardia cm; Lam., or 

 rather of Isocardia Moltkiana, Lam., having a notable angle on the umboniai slope 

 and presenting a perfectly cordate form on the anterior view. The posterior cicatrices 

 are confluent like Monocondijlcea, and but slightly impressed. The inferior anterior 

 cicatrices are deeply impressed, and so closely united as scarcely to be distinguishable. 

 The superior anterior cicatrix is placed on the inner side of the cardinal tooth, and 

 higher up than on that of the Monocondyloea. The dorsal cicatrices are placed so far 

 into the deep cavity of beaks, as to be but partly and faintly visible. 



The absence of the soft parts of this shell is greatly to be regretted. From the 

 globular form, the epidermis and the teeth, we would be led to expect to find some- 

 thing different in the structure of some of its organs, as it is very likely to be in its 

 habits, from Mojiocondyloea or Vnio. 



Judging from a very imperfect left valve of Monocondyloea Quarayana, D'Orb., all 

 that I have seen, I should not be surprised if it would prove to belong properly to this 

 subgenus. Certainly the form of tooth is different from that of the other Monoco?idyI<xa. 

 It is, however, very much compressed, and is much thicker in the substance of the 

 shell. 



The epidermis of Plagiodon is of that texture and color, which reminds one of 

 some of the estuary genera, Cyrena, Clathrodon, &q., but its pearly nacre and form 

 of teeth place it among the Naiades without doubt. 



Plagiodon isocardioides. PI. 32, fig. 32. 



Testa laevi, rotundo-trigona, globosa, ventricosissima, ad lateris planulatis, valdfe inaequilaterali, antice 

 brevissima, umbonibus tumidis, valvulis subtenuibus ; natibus valde elevatis, incurvis terminalibusque ; 

 epidermide subrugosa, tenebroso-ollvacea ; dentibus cardinalibus crenulatis, compressis, transversis, 

 curtis in utroqu^ valvulo duplicibusque ; raargarita alba et iridescente. 



Shell smooth, subtriangular, globose, very much inflated, flattened at the sides, very 

 inequilateral, truncate before, swollen on the umbones ; valves rather thin ; beaks 

 very much elevated, incurved, terminal ; epidermis dull, dark olive ; cardinal teeth 

 crenulate, compressed, transverse, short, double in each valve ; nacre white and 

 iridescent. 



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

 Hab. — Rio Plata. Mr. E. Verreaux, Paris. 



My cabinet and cabinet of Mr. Verreaux. 

 Diam. -9, Length -9, Breadth 11 inch. 



Shell smooth, subtriangular, globose, very much inflated, flattened at the sides, very 

 inequilateral, truncate before, subemarginate, swollen on the umbones ; substance of 

 the shell rather thin, thicker before ; beaks very much elevated, incurved and 

 recurved ; ligament very small, nearly black, thin and not raised alcove the plane of 



