PLACUNANOMIA. 



Plate I. 



Genus PLACUNANOMIA, Broderip. 



Testa suiaquivalvis, irregularis, tenuis, interdimi soUdius- 

 cula, plerumque complauata, cardine interno dmiihus 

 duobus divaricati^ coiivergerdUms hi valvd u/feriore, 

 sulcis duobtis ligument'iferis in superiore. Valva infe- 

 rior prope cardinem fismrata vel trigono-perforata, 

 perforatione cardiiie descendente, tendinem adhasloiiis 

 subosseam interdum inter testa laminas insertam, ad- 

 mittente. Impressio mmcularis in valvd inferiore uni- 

 ca, in superiore dupUcata, subcentralis. 



Shell nearly equivalve, irregular, thin, sometimes rather 

 solid, generally smooth ; hiuge internal, composed in 

 the lower valve of two divaricate, converging teeth, 

 in the upper of two ligamentary grooves. Lower 

 valve fissured or triangularly perforated near the 

 hinge ; perforation descending from the hinge, admit- 

 ting a partially bony tendon of adhesion, wliioh be- 

 comes sometimes inserted between the lamina; of the 

 shell. Muscular impression in the lower valve single, 

 in the upper valve double, nearly central. 



This remarkable genus was founded in 1S32 by Mr. 

 Broderip, on a single species collected by Mr. Cuming iu 

 the Gulf of Dulce, Central America, presenting a hinge 

 approaching very nearly to that of P/acuna, in connection 

 with the appearances and distinguishing organization of 

 Anomia. As many as a dozen other species have been 

 found to partake of these typical characteristics, but iu 

 none are they so conspicuously developed as iu the origi- 

 nal Placunanomia Cumingii. Their generic peculiarity con- 

 sists in this : the orifice in the lower valve, through which 

 the ossified tendon of adhesion, or plug, passes, is not 

 open at the top as in Anomia, but is soldered (so to 

 speak) by the intervention of a divaricate, Placuna-Vikt 

 hinge, or by some more or less rude modification of it. 

 Owing to the soldering of the orifice, the tendon of adhe- 

 sion is so far restricted iu its growth as to insinuate itself 

 in some species in the form of a plate between the lamina; 

 of the surrounding shell ; this is not, however, as Mr. Bro- 

 derip had reason to suppose from the P. Cumingii, a cha- 

 racter of the genus. In most of the species the orifice is 

 large enough to admit the utmost growth of the tendon. 



The species, it will be seen, are from widely-spread lo- 

 calities, one of which inhabits our own shores. 



Species 1. (Mus. Cuming.) 

 PLACUN.4N0MIA ECHINATA. Plac. testd subtumidd, v.ai- 

 vd snperiore radiatim echinato-sqtiamatd ; albidd^ me- 

 dio fusco tinctd. 

 The prickly Placunanomia. Shell somewhat swollen, 

 upper valve radiately prickly-scaled ; whitish, stained 

 in the middle with brown. 

 BuoDERip, Pro. Zool. Soc. 1834, p. 2. 

 Hab. Island of Nevis, West Indies (dredged up attached 

 to shells from sandy mud, at a depth of six fathoms); 

 Mr. Powers. 

 Dr. Gray regards this species as an imperfect specimen 

 oi P.foUata, and states in his monograph of the genus. 

 Pro. Zool. Soc. 1849, p. 121, that " Mr. Broderip doubted 

 if this might not be the case when he described it." It 

 raiiy be so, but I cannot find that Mr. Broderip has made 

 any such statement. The P.fuUuto has a much Inrgei' 

 orifice for the tendon of adhesion. 



Species 2. (Mus. Cuming.) 



Placunanomia rudis. Plac. testd sordide alba, crassd, 

 concenirice irregulariter corriigatd, intua politd, per- 

 foratione parvd. 



The rude Placunanomia. Shell dead-white, thick, 

 concentrically irregularly wrinkled, polished within, 

 orifice small. 



BiiODERip, Pro. Zool. Soc. 1834, p. 2. 

 Pododesmus decipiens, Philippi. 



Ilab. West Indies; Broderip. Havana; Philippi. 



Of rude growth and solid substance, with the orifice 



Species 3. (Fig. a, b, Mus. Cuming.) 



Placunanomia Cumingii. Plac. testd subrotundatd, ob- 

 scure argenteo-albidd, virescenie-tinctd, complanata, 

 margine pUcatd, plicis ma.nme angularibus. 



Cuming's Placunanomia. Shell somewhat rounded, 

 obscurely silvery-white, tinged with green, smooth, 

 plaited at the margin, plaits largely angular. 



Broderip, Pro. Zool. Soc. 1832, p. 29. 



Hab. Gulf of Dulce, province of Costa Rica, Central Ame- 

 rica (dredged from a muddy bottom at a depth of 

 eleven fathoms, attached to dead bivalve shells and 

 dead coral); Cuming. 



August, 1859. 



