TO GEOLOGY. 81 



direct ; cavity of the shell deep ; margin very minutely 

 crenulate ; nacre pearly. 



Diam. .3, Length .4, Breadth .5, of an inch. 



Observations. This pretty little species in general ap- 

 pearance resembles the Sedgewickii, but differs in being 

 more ovate and in having a lunule. Of the species de- 

 scribed by Mr Sowerby, it approaches most closely to the 

 variabilis, but differs in not being smooth outside, and in 

 having a crenulated margin. 



JV. pectuncularis. Plate 3. Fig. 60. 



Description. Shell trapezoidal, compressed, nearly equi- 

 lateral, angular before, rounded behind, flattened on the 

 sides, straight at basal margin, longitudinally and minute- 

 ly ribbed ; anterior slope large, carinate ; substance of the 

 shell thin ; beaks recurved, pointed ; both series of teeth 

 small and nearly equal, the posterior one rather the smaller ; 

 fosset on the plate and immediately under the beak, ob- 

 lique ; cavity of the shell shallow ; margin entire ; nacre 

 apparently not pearly. 



Diam Length .1, Breadth 3-20ths, of an inch. 



Observations. A single valve only of this truly inter- 

 esting little nucula has been obtained by me. It differs 

 in outline from any species I am acquainted with, its 

 trapezoidal form being very peculiar. At first sight, on 

 examination of the teeth, I took it to be a Pectunculus, 

 but a further examination with the microscope showed me 

 the fosset for the ligament, between the two series of 



L 



