64 CLASS I. TROPIOPODA. ORDER I. B1MUSCULOSA. 



The genus Mesodesma, which we gladly adopt, to facilitate the arrange- 

 ment of the Mactracea, was instituted by Deshayes for the purpose of 

 associating those species which are intermediate in their characters be- 

 tween the Crassatellee and the Amphidesmata. They may certainly be 

 said to approximate rather to the former, but are still clearly distinct ; 

 and the propriety of this genus has moreover been fully confirmed by a 

 careful investigation of the animal, since made by M. Quoy. 



It may be observed, on reference to our synonyma, that some of the 

 Mesodesmata have been inadvertently taken by Sowerby as examples of 

 Lamarck's genus Erycina, a genus, which the former, like most natural- 

 ists, was evidently at a loss to identify. Lamarck might well have pro- 

 nounced his Erycina to be " des coquilles en quelque sorte equivoques," 

 he, however, described but one recent species ; and although Deshayes 

 uncourteously notices in his 'Description des Coquilles Fossiles,' that La- 

 marck associated in his genus Erycina " presque toutes les petit es coquilles 

 dont on n'a pas Men reconnu le genre," he has not failed to acknowledge 

 a second recent species, described by Payrandeau in his ' Catalogue des 

 Mollusques de l'lle de Corse.' Neither of them are we able to identify, 

 and have therefore abandoned Lamarck's genus Erycina altogether; still, 

 whether it may be preserved with propriety as a fossil genus or not, it is 

 not our province to determine. 



The shell of Mesodesma may be described as being ovate, transverse 

 or somewhat triangular, equivalve, rather compressed, thick and gene- 

 rally closed. The hinge has two thick lateral teeth in each valve, be- 

 tween which is situated a deep spoon-shaped cavity for the insertion of 

 the ligament. The muscular points of attachment are unequal, and the 

 impression of the mantle exhibits a sinus on the posterior side. 



Examples. 



PI. XLV. Fig. 1. 



Mesodesma Chemnitzii, Deshayes, Enc. Meth., vol. ii. p. 443. Quoy 



et Gaimard, Voy. de l'Astrolabe, pi. 82. f. 9 to 11. 

 My a Australia, Gmelin. 

 Mya Nova Zelandice, Chemnitz. 



