228 BULLETIN OF THE 



Shell inequilateral, inequivalve, closed ; upper or left valve slightly smaller, 

 lower or right valve attached to some extraneous object; external layers 

 nacreous ; inner surface porcellanous ; epidermis none or very little ; liga- 

 ment linear, minute; cartilage inserted in a triangular pit in the cavity of 

 the beak; hinge-line short, straight; inner margins radiatingly wrinkled ; pallial 

 line simple. Mantle completely open, margin papillose without ocelli; gills 

 single, one on each side composed of a single row of long filaments, palpi 

 none; anal end of intestine produced, free; sexes separate; foot none; anterior 

 adductor single, distinct ; posterior adductor double, and leaving a pair of 

 closely approximated subequal impressions on the shell ; mouth with distinct 

 lips ; visceral mass smalL 



Dimya Deshayesiana RonACLT. 



Dimya Deshayesiuna Rouault, 1. c, p. 471, pi. xv. figs. 3, 3 a, 8 b, 1848. Eocene of 

 Bos d'Arros, France, equivalent in age to the Paris Basin eocene. 

 Stoliczka, 1. c, p. 397, 1871. 

 Anomia intustriata D'Archiac, Mem. See. G^ologique de France, 2me ser., III. p. 441, 



pi. xiii. figs. (9 a, 10 a?), 11, 1848.* 

 Dimya Deshayesiana Tate, Woodward, Man. Moll, 2d ed., p. 408, 1871. 



Dimya argentea Dall. 



Plate IT. FigB. 5 a, 6 b. 



Shell white, micaceous silvery outside, opaque brilliant porcellanous white 

 inside ; irregular, laterally compressed, attached by the beak of the right valve 

 (to a dead echinus-test, etc.), which is deeper and larger than the other ; ex- 

 terior obscurely finely radiately striate ; outline irregularly ovate, broader be- 

 hind ; hinge-line short, straight, without notch or auricles ; in well-developed 

 sxamples it has a leaf-shaped wrinkled area on each side of a small impressed 

 triangiilar area, below and partly under which is a small, deep, subtriangular 

 pit for the brown homy cartilage ; ligament hardly perceptible, linear, nearly 

 as long as the hinge-line; interior with an impressed area bounded by the 



* It se^ms very probable that the shells described as Anomia by D'Archiac were 

 all of the same species as that subsequently described by Rouault. Fig. 11 cer- 

 tainly represents the same shell, and it looks as if Figs. 9a and Ida represented 

 attached valves of Dimya which had been worn through at the point of attach- 

 ment of the lower valve, and the resulting accidental perforation taken to be 

 normal by D'Archiac. The interior markings are precisely similar, as far as can 

 be judged from the figures. In the absence of specimens, however, it is safer to 

 preserve the specific name of Rouault, which certainly relates exclusively to the 

 species as we now understand it. Rouault's paper was read in 1847, but seems to 

 have been published in 1848, while .the volume appeared as a whole in 1849. 



