160 EOCENE MOLLUSCA. 



a fulcrum. The dentition, also, in digitaria is different, there being in the right valve of 

 that shell one cardinal obtuse and truly triangular tooth, and tvv^o elongated and distinct 

 and distant lateral teeth, with a similar arrangement of denticles in the left or opposite 

 valve, so that the prominent parts of the hinge of the one interlock into the corresponding 

 depression of the other; but in the Eocene shells there are no lateral teeth, only one 

 large bifid or diverging cardinal tooth — or, I ought rather to say, they are two distinct 

 diverging teeth in the right valve beneath the umbo. There are no oblique markings 

 upon the exterior of the older Tertiary shells — only crenulations upon the inner margins 

 of the valves, which diverge or run out in opposite directions ; but in digitaria those 

 oblique markings are wholly confined to the outer layer of the shell, and are not 

 impressed upon the inner margin, — corresponding in that respect with those species of 

 Asiarte in which the exterior is ornamented with oblique ridges. 



In the * Journ. de Conch.,' vol. x, p. 141, M. 0. Semper has enumerated nine species 

 as belonging to this genus; and these he has divided into three groups, viz., 1st, especes 

 striees, four species; 2ndly, especes lisses, four species; and 3rdly, especes inequilaterales, 

 surface striee oii treillissee. With the exception of digitaria, Linn., they are all fossil. 



1. WooDiA CRENULATA? Desliages. Tab. XXV, fig. 11. 



WooDiA CBENULATA (?), Besh. All. sans Vert, du Bass, de Par., t. i, p. 792, pi. lix, 



figs. 9—11, 1860. 



Spec. Char. " W. Testa minima, orhiculari, solidula, convexiuscula, lavigata, sub- 

 cequilaterali ; umhonibus tumiduUs, oppositis ; marginihus lateralihus late et oblique crenu- 

 latis, margine inferiore tenuissime crenidato, dente cardinali unico in valvula dextra postice 

 elongato ; dentibus diiobus in valvula sinistra subcsqualibus." — Deshayes. 



Shell small, orbicular, strong, slightly convex, smooth, nearly equilateral ; umbones 

 shghtly prominent, inner margins broadly and obliquely crenulated ; one cardinal tooth 

 in the right valve, and two diverging teeth in the left. 



Diameter, \\\\. of an inch. 



Locality. Barton {Edwards). 



A single specimen of this genus is all that I have seen from the English Lower 

 Tertiaries ; it is in Mr. Edwards's cabinet, and it has affixed to it the MS. name of 

 W. orbicularis, from which I presume that gentleman considers it to differ from the 

 French shell. I feel unwilling, however, to found a new species upon the minor 

 differences here presented ; and it will require a comparison with the type specimen of 

 M. Deshayes before a separation could be justified, and this I am not able to make. 

 Our specimen is the right valve, and the cardinal "bifid denticule" is, in fact, composed 

 of two teeth, diverging from beneath the umbo at an angle of 75°. These teeth are of 

 unequal size and length, with a distinct separation between them : there are no lateral 



