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the hinge on a straight line ; Pectunculus, having the hinge arched ; and 
Nucula, having the hinge on a line bent at an angle. The shells of this 
genus are marine shells. 
Nucula margaritacea, Lam. Area nucleus, Linn, are found at Grignon, 
Courtagnon, &c. I have also found shells of this species, with their fine 
comb -like teeth, and their pearly coat, quite perfect, in the Essex bank 
of shells ; and in a perfect state, and of a microscopic size, at Plumsted. 
I have also disengaged one or two minute calcedonic specimens of this 
delicate shell, in a perfect state, from the Devonshire whetstone. 
Lamarck describes two more species of this genus among the Parisian 
fossils ; N. striata and N. deltoidea. In a minute fossil-shell of this last 
species, not larger than a small pin’s head, which I found in a Cerithium 
gigas, the original margaritaceous lustre still exists ; and in one of the 
former, of the same size, the strife are very evident. 
XCVIII. Pectunculus. An orbicular, subequilateral bivalve, with an 
arched hinge ; with numerous teeth, alternately inserted in a single row. 
Lamarck enumerates five species : 
P. angusticostatus. P. terebratularis . P. nuculatus. 
P. puloinatus. P. granulatus. 
The shells of this genus are easily known by their rounded or lenti- 
cular form. Their teeth are larger, and less closely set, than those of 
the arks, and disposed in an arched line, which becomes very narrow, or 
is even interrupted, under the beaks. The muscular impressions are 
two, and strongly marked ; each forming a callous projection, the edge 
of which is sharp. 
In the Essex bank, numerous shells of this genus are found, which 
seem exactly to agree in character with P. glycemeris , Lam. Area gly- 
cemeris , Linn. The species of this genus are, from their general agree- 
ment, difficult to separate ; but I believe that the skilful conchologist 
would be able to make further divisions of these Essex shells. Shells of 
this genus, and chiefly, I believe, P. angusticostatus, Lam. are frequent 
in the Bognor rocks. 
VOL. III. 
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