NEW EXOTIC UNIONID.E. 390 



Shell smooth, very wide, compressed, emarginate, flattened at the side, inflated 

 before, widened and flattened out behind ; valves very thin, diaphanous ; beaks 

 small and a little prominent; epidermis yellowish horn-color, very much striate, 

 shining, eradiate ; nacre bluish-white and very iridescent. 



Hah.— Siam, S. R. House, M. D. 



Cabinet of Mr. Haines. 

 Diam. "6, Length 1-2, Breadth 4-9 inches. 



Shell smooth, very wide, compressed, emarginate on the anterior basal margin, 

 flattened on the sides, inflated rather suddenly anteriorly from the beaks oblique- 

 ly to the basal margin, widened and flattened out behind ; substance of the shell 

 very thin, translucent; beaks small, a little prominent and pointed; ligament 

 very long, thin "and dark-brown; epidermis yellowish horn-color, very much striate, 

 shining and without rays; umbonial slope slightly raised and rounded; posterior 

 slope much compressed, raised into a high keel, with an indented line from the 

 beak to the posterior margin in both valves; dorsal line furnished with a slight, 

 long rising, almost amounting to an acicular tooth ; anterior cicatrices large, indis- 

 tinct, apparently confluent ; posterior cicatrices large, confluent and very indistinct ; 

 cavity of the shell shallow and very wide ; cavity of the beaks very shallow ; nacre 

 bluish-white and very iridescent. 



Remarks. — This very remarkable shell was submitted to me by Mr. Haines, who 

 has done so much in bringing to light the Siamese and other Molluscs. I had 

 doubts, on first receiving it, whether it, could be from the East, as we have not be- 

 fore seen a Mycetopus out of South America, — considering the genus to belong ex- 

 clusively there. Mr. Haines, however, informs me, on reviewing the matter, that it 

 came from Dr. House, who has sent him so many new river Molluscs which I have 

 described in a previous volume. We have in this species a character which I ha"ve 

 never before observed in any one of the family, — an enlargement, or inflation of 

 the anterior fifth of the valves, commencing at the beaks and inclining obliquely 

 to the anterior basal margin, which is emarginate and gaping. A disposition to 

 putting on a long, acicular lateral tooth in each valve, presents the first time sucli 

 a case in any Mycetopus I have seen. In the left valve, immediately under the beak, 

 there is an indistinct callus, resembling an incipient tooth ; but this may be acci- 

 dental. That portion of the valves which is not inflated has a very strong resem- 

 blance to M. siliquosus, D'Orb. 



Monocondylojia Wheatleyi. PI. 50, fig. 307. 



Testa lijevi, oblouga, subcoinpressa, valde iuEequilaterali, antice oblique rotundata, postice truncata ; valvulis 

 subcras.sis, antice aliquanto crassioribus ; natibus parvis, acumiuatis. ad apices minutissime uudulatis ; 

 opidermidc lutea, Ditida, eradiata ; dentibus cardinalibus parvis. orcctis. in utroque valvulo uno- 

 tubcrculatis ; margarita alba et valde iridcsceatc. 



