Mesodestna.] PELECYPODA. '.157 



Genus 1. MESODESMA, Deshayes, 183J. 



Mesodesma, Deshayes, Ency. Meth. Vers. ii, 1831, 441. Type: Mactra 

 donacia, Lamarck. 



Animal with the borders of the mantle smooth ; siphons cylindrical, 

 distinct, long, the anal siphon with conic tubercles, the branchial 

 pinnate ; foot long, laterally compressed, triangular, pointed ; palps 

 triangular ; gills very unequal. 



Shell subtrigonal, depressed ; anterior side the longer ; hinge 

 thick, carrying a spoon-shaped depression for the ligament (resilium), 

 in the centre, a small bifid or double cardinal tooth in front of the 

 ligament - pit ; anterior and posterior lateral teeth double on the 

 right valve, single on the left ; pallia! sinus more or less deep. 



Distribution. Mediterranean, east coast of North America, west 

 coast of South America, Indian Ocean, Philippines, Australasia. 



Fossil in the Tertiary. The genus is represented in the Eocene 

 of Australia, and most likely migrated from there to New Zealand. 



KEY TO SUBCJENERA. 



A. Shell triangularly oblong ; anterior end longer. 



a. Pallial sinus shallow, rounded ; ligament small, obsolete . . DONACILLA. 

 aa. Pallial sinus reaching to near the centre, rounded ; liga- 

 ment short, strong, partly external . . . . . . TABIA. 



B. Shell oblong-oval, the beaks at or near the middle ; pallial sinus 



small, angular ; ligament small, almost wholly internal . . PAPHIES. 



Subgen. 1. DONACILLA (Lam.), Philippi, 1836. 



Donacille, Lamarck, Extr. d'un Cours, 1812, 107. Donacilla, Philippi, Moll. 

 Sicil., 1836, 37. Amphidesma sp., Lamarck, 1818. Mesodesma sp., Des- 

 hayes, 1830. Erycinn sp.. Sowerby, 1822. Type: Mesodesma rorneum, 

 Poli ; Mediterranean. 



Ligament marginal, obsolete ; laterals not sulcate. the anterior 

 lateral long, the posterior short, stout, triangular, with the posterior 

 margin of the cartilage-pit raised and thickened like a second tooth ; 

 posterior ventral lamina in the right valve triangular, stout, elevated, 

 vertically directed, the dorsal lamina obsolete, the anterior laminae 

 elongated. 



Distribution. Europe and Australasia. 



I. Mesodesma subtriangulatum, Gray, 1825. Plate 59, tig. 19. 



Erycina subtriangulata, Gray, Thomson's Ann. Philos., xxv. 1825. Meso- 

 desma subtriangulata, Gray, Dieff. N.Z., 252. Afactodea subtriangulata, 

 Gray, Index, 91. Mesodesma lot um, Deshayes. in Guerin's Mag. Zool.. 

 1843, 80 ; Conch. Icon., 1854, 1. 4 ; C.M.M., 68 ; Grit. List, 43 ; Hutton, 

 L de Conch., xxvi, 48 ; M.N.Z.M., 146. M. */</*/, Reeve, Conch. Icon.. 

 1854, f. 18 ; C.M.M., 68 ; Grit. List, 43 ; Hutton, J. de Conch., xxvi, 48; 

 M.N.Z.M., 146. Paphia spissa, Reeve : Hutton, P.L.S. N.S.W., ix, 519. 

 Mesodesma cuneata, Lamarck : Hutton, G.M.M., 68 ; frit. List, 43, not 

 of Lamarck. 



Shell moderately large, triangularly oblong, very inequilateral, 

 solid, posteriorly truncated almost smooth, polished, compressed. 



