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 alunule. 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. 
N. 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 
bi 
