1(39 



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 Plurnsted. 

 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 A r . ddtoidea. In a minute fossil-shell of this last spe- 

 cies, not larger than a small pin's head, which I found in a Ceritkium 

 gigds, the original margaritaceous lustre still exists ; and in one of the 

 former, of the same size, the striae 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. pulvinatus, P. 

 terebratularis, P. granulatus, P. nuculatus. 



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 beuks. 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. elvcemeris, Lam. Arca&bN- 



v O C* , -^ ./ 



cemeris, Linn. The species of this genus are, rrom their general agree- 

 ment, difficult to separate ; but I believe that the skihul concholo^jst 

 would be able to make further divisions of these Essex she j is. Sheik of 

 this genus, and chiefly, I believe, P. august icostaius, L^m. are freqpent in 

 the Bognor rocks. 



In the whetstone sand-pits, at Blackdown, a small species of this ge- 



VOL. III. Z 



