36 EOCENE MOLLUSCA. 



Shell ovately oblong, elevated, and somewhat tongue-shaped, depressed and inequivalved, 

 tumid at the upper part, with very divergent umbones ; shell subnacreous, irregularly smooth 

 with conspicuous lines of growth and a sharp margin, base prominent and rather pointed ; 

 muscular impression elongated. 



Longest diameter, \\ inch. 



Localities. Barton {Edwards). 



France — Grignon, Chaumont, Mouchy, le Chatel {Desk.), Blaye, {Gratel). 



The hinge-area in the British fossil has apparently a larger ligamental depression, and 

 a smaller sinus than is represented in the French shell ; but these differences would 

 probably vanish on a comparison with specimens of the shells themselves. The connexus 

 is somewhat on one side of the hinge-area, as if counteracting the action of the adductor 

 muscle which is on the other. The left valve extends the ventral margin beyond the edge 

 of the right, making the shell inequivalve, like the oyster. 



The above name has been published for an English fossil, and, as I have not the means 

 of disproving the identity, I have permitted it to remain with a mark of doubt. The shell, 

 like most of the oyster tribe, has a tendency to great variation, and the muscle-mark 

 partakes of the varying form of the shell. Since the above was written, I find the follow- 

 ing observation by M. Deshayes, in ' Hist, des An. sans. Vert du bassin de Paris,' p. 51 : 

 — " L'espece est citee dans le bassin de Londres par M. Morris ; mais a Barton elle est 

 accompagnee d'une autre tres-distincte, beaucoup plus rapprochee par ses characteres d'une 

 espece vivante de la Mer Rouge, et d'une autre fossile du terrain nummulitique de PInde 

 que M. d'Archiac a fait connaitre sous le nom de Vulsella lignium." 



I have made every effort to see the specimens here referred to, but without success. 

 I am not aware that the genus has been found in the London basin, or anywhere but at 

 Barton ; and all the specimens that I have seen may, in my opinion, be referred to one 

 species, but whether that be the true deperdita I cannot say. 



PECTEN. Pliny, Aldrovandus, &c. 



Ostrea (sp.). Linn. — P. opercularis, Sect. a. 

 Hinnites. Sow. (not Defr.). — P. pusio, „ b. 

 Jantra. Schum. — P. maximus, ,, c. 



Neithea. Drouet. — P. quinquecostatus, „ d. 

 Pleuronectia. Swains. — P. pleuroneetes, „ e. 



Gen. Char. Shell lenticular or sub-orbicular, sometimes ovate, sub-equilateral, gene- 

 rally inequivalve ; ornamented with radiating stria? or more or less elevated ribs : beaks ap- 

 proximate and acute, with a projecting and unequal auricle on each side of the umbo ; 

 pedal region slightly sinuated, right valve generally the more convex ; connexus bipartite, the 

 ligamental portion narrow on the straight hinge-margin, cartilage placed in a central pit, 



