412 MACTRIDJ5. 



however, a numerous and widely spread group: its 

 favourite habitat is sand or mud. Dr. Carpenter has 

 ascertained that the texture of the shells presents the 

 same general characters as in the Tellinida, but that the 

 indications of organic structure are more distinct. In 

 those of Mactra and Lutraria the cells are especially 

 observable, although irregular in the one and elongated 

 in the other. 



Tier-Ton 

 Genus I. AMPHIDES'MA*, Lamarck. PL VIII. f. 1. 



BODY oval,* compressed : mantle thick, united for two- thirds 

 of its length : tubes separate and unequal, not very long, and 

 having their orifices fringed with cirri. 



SHELL oval, only slightly inequilateral, wedge-shaped, closed 

 at both ends, longitudinally as well as concentrically striated 

 in some species : beaks turned towards the posterior side : teeth, 

 two cardinals, of unequal size, in each valve ; laterals more or 

 less distinct. 



Although, as Lamarck observed, the Amphidesmatd 

 in the aggregate appear to form an artificial group 

 and, as originally constituted, they were truly a hete- 

 rogeneous assemblage of species the characters of this 

 genus in a restricted sense are peculiar, and it serves to 

 connect the present with the immediately preceding 

 family. In the outward form of the shell (as well as 

 in the siphonal tubes) it resembles Donax, and in the 

 structure of its hinge Mactra. Whether the genus 

 Mesodesma of Deshayes is different may be an open 

 question ; but there is much less cause to separate Tur- 

 ton's genus Ervilia, which not only was very inade- 

 quately and inaccurately defined, but did not even in- 

 clude the only British species lately assigned to it by 

 Forbes and Hanley, and which will be now described. 

 * Having a double ligament. 



