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 



