PLACUNANOMIA. 



TESTA adhserens, subaequivalvis, irregularis, 

 coinplanata. Cardo dentibus duobus divarica- 

 tis, basi convergentibus in valva inferiore, sul- 

 cis duobus ligamentiferis in superiore. Valva 

 inferior cardinem versus fissurata, organo ad- 

 hsesionis subosseo inserto, fissuram implente. 

 Impressio muscidaris in utraque valva subcen- 

 tralis In valva superiore organi adhaesionis 

 impressio superaddita. 



This interesting Genus partakes of the characters of the 

 genera Ostrea, Plicatula^ Placuna and Anomia. It may 

 he regarded as the connecting link between the two latter. 

 With an arrangement of the hinge approaching very 

 nearly to that of Placuna we have the distinguishing or- 

 ganization of Anomia, while the external appearance of 

 the shell, especially if viewed in situ, bears the strongest 

 resemblance to a Plicatula, or some of the plicated Oys- 

 ters. The organ of adhesion, which in its bony character 

 (for it is rather bony than shelly) resembles that of Ano- 

 mia, does not perforate the lower valve directly, but is 

 inserted between the laminae of the internal surface of 

 the lower valve above the muscular impression, (for Pla- 

 cunanomia has but one in each valve) and below the hinge 

 and passes out into an external, irregular, somewhat lon- 

 gitudinal superficial fissure or cicatrix, narrowest at the 

 hinge margin and which it entirely fills to a level with the 

 surrounding surface of the shell. 



We have met with three species of this curious Genus, 

 that which appears to be typical was brought by Mr. Henry 

 Cuming from the Gulf of Dulce in the Province of Costa 

 Rico: it has been named P. Cumingii, and our representa- 

 tions are taken from two of his specimens. Of the other 

 two species one is from Lu9on, one of the Philippine 

 Islands, the other has been occasionally found adhering 

 to Madrepores in the West Indies. 



