the flat valve; and by four diverging raised 'teeth on the convex valve; 

 the latter being so disposed, and so crenulated on their inner surfaces, 

 as exactly to receive, as in the Trigonia, the teeth of the opposite valve. 



Aware of the danger of unnecessarily multiplying genera, I was at 

 first disposed to consider this shell as a species of Trigonia : from this, 

 however, I was precluded, on discovering that, unlike the shells of that 

 genus, these were adherent a difference, undoubtedly, sufficient to pre- 

 vent their being disposed under the same genus. 



Plate XII. Pig. 14, represents the internal surface of the flat valve; 

 and Fig. 16, a magnified view of its hinge-teeth. Fig. 15 is the internal 

 surface of the convex valve, and Fig. 17 a magnified view of its hinge- 

 teeth. Fig. 18 is a magnified view of the outside of the flat valve, showing 

 those strong hooked instruments, from which it has been presumed to 

 give a name to the genus. One specimen of the upper valve of this 

 shell manifests a very considerable degree of lustre. 



CXLV. Pecten. A regular, eared, inequivalved bivalve, with conti- 

 guous beaks. The hinge toothless, the pit trigonal, receiving the inter- 

 nal ligament. One muscular impression. 



Of those which are in my collection, I much regret the being totally 

 uninformed of the locality of most of them. This I particularly regret 

 being the case with a specimen w r hich has been imbedded in a hard 

 whitish lime-stone, and which is in every respect analagous with P.jaco- 

 beus, List. Conch. 165, /. 2, 166, /. 3, having, on the lower valve, four- 

 teen angulated rays, longitudinally striated : the sulci transversely stri- 

 ated : the upper valve having the rays rounded and striated transversely. 

 Thus is it also with a most perfect specimen of P. radida and P. varius, 

 differing in no respect from O. radula and O. van/i. Linn, except in each 

 being of a small size. 



Lamarck particularizes three species found in the neighbourhood of 

 Paris : P. plebeius, P. infumatus, P. squamula. In the Harwich cliff are 

 specimens found, which I think are referable to the two former of these 

 species. 



