MESODESMA. 



Plate I. 



Genus MESODESMA, Deshmjes. 

 Testa trigono-ovata vel transversa, interdum trigono-lanceo- 

 lata, plus niinvs compressa, fnquivalvis, iniequilateralis, 

 plerumque alba, pariim colorata, parum exscnlpta. 

 Cardo dentibua lateralibus dttobus, crassis, in utrdque 

 valm, cmn. foveold profimdd cochleariformi interja- 

 cetite, liganiento inserto. Impressiones nmsculares in- 

 aquales, anteriori majore, elongatd ; impressio pallii 

 simi posteriori. 

 Shell tj-iangularly ovate or transverse, sometimes triangu- 

 larly lanceolate, more or less compressed, equivalve, 

 inequilateral, generally white, little coloured, little 

 sculptured. Hinge with two thick lateral teeth in 

 each valve, with a deep spoon-shaped pit lying be- 

 tween, with the ligament inserted. Muscular im- 

 pressions unequal, anterior the larger, elongated ; 

 impression of the mantle with a posterior sinus. 

 The genus Mesodesma was instituted by M. Deshayes 

 for the reception of certain species of Mactracea having 

 distinct peculiarities of their own, that had been assigned 

 without consideration to different allied genera. They 

 include Mactra donacia, Crassatella polita, Crassatella 

 cuneata, Crassatella striata, Crassatella eryciruBa, AmpJii- 

 desma donacilla, and Ampjiidesma 'glabrella of Lamarck, 

 Mya Novce-Zclandia of Chemnitz, and Erychia complanata 

 of Sowerby. They are of a colourless heavy texture, and 

 are mostly characterized by a triangular wedge form, 

 though some are transverse and suborbicular. The hinge 

 has not the thin triangle tooth of Mactra, and the liga- 

 ment, unlike that of Crassatella, is situated in a spoon - 

 shaped cavity alongside the cardinal teeth. The muscular 

 impressions are unequal, the anterior being elongatfxl, and 

 the impression of the mantle is deeply sinuated on the 

 posterior side. 



To the ten species described by M. Deshayes in his edition 

 of the 'Anim. sans vertebres,' we have to add twenty-one 

 species, described chiefly by him about a twelvemonth since, 

 in the collection of Mr. Cuming. They are intended for 

 publication in the ' Proceedings of the Zoological Society ' 

 and in the ' Catalogue of MoUusca in the British Museum ;' 

 but as in neither of those works have the sheets containing 

 Mesodesma yet issued from the press, I can only refer to 

 M. Deshayes' manuscript tickets in the Cumingian collec- 

 tion, where the monograph was prepared. 



The geographical distribution of the genus is very gene- 

 ral. The most northern species are two which inhabit the 



shores of Massachusetts, and one small species in the Me- 

 diterranean. New Zealand, Australia, and Van Diemen's 

 Land contribute more than half of the species of which the 

 localities are known. One single species, inhabiting Chili, 

 is all that we have from South or Central America. 



The most abnormal form of Mesodesnm is M. triquetra, 

 Hanley, which is Mr. Gray's genus Anapa. 



Species 1. (Mus. Cuming.) 



Mesodesma coenea. Mes. testa trigono-oblongd, crassd, 

 lavigatd, hdescente-albd, quasi pellucidd, radiis duabus 

 nigricantibus notatd ; lateribus rotundatis, antko bre- 

 vissimo. 



The hoeny Mesodesma. Shell triangularly oblong, 

 thick, smooth, yellowish -white, as though trans- 

 parent, marked with two blackish rays ; sides 

 rounded, anterior very short. 



Deshayes, Anim. sans vert. vol. vi. p. 133. 

 AmpMdesma cornea, Lamarck. 

 Mactra cornea. Poll. 

 Ampliidesma donacilla, Lamarck. 

 Bonax plebeia, Montagu. 

 Erycina plebeia, Sowerby. 



Hab. Mediterranean. 



A rather solid though subtransparent-looking species, 



mostly rayed from the uraboes with two grey-black bands. 



Species 2. (Mus. Cuming.) 



Mesodesma teansversa. Mes. testa trigono-oblongd, 

 compressiusculd, STibepqiiilaterali, antice breviore, lavi- 

 gatd, sub lente siibirregidariter concentrice striata; pel- 

 lucido-albidd, niteyite, lidescente tinctd. 



The transverse Mesodesma. Shell triangularly ob- 

 long, rather compressed, nearly equilateral, ante- 

 riorly shorter, smooth, beneath the lens somewhat 

 irregularly concentrically striated ; transparent white, 

 shining, tinged with yellow. 



Deshayes, MS. (Mus. Cuming.) 



Hab. ? 



A subtransparent shining yellow-tinged species, of less 



solid texture than the preceding, obscurely concentrically 



striated. 



July. 1854. 



