165 
not possessing transparency, at Montmorency, Erappes, &c. Lamarck 
observes, that it has the form of a very small seed of some species of 
lucern ; and, hesitating at determining it to be really a multilocular 
shell, only assumes it as such for the present. At Fig. 24 is repre- 
sented one of its detached carinated ribs. 
LETTER XIII. 
BIVALVES WITH EQUAL VALVES, AND REGULARLY FORMED PINNA 
MYTILUS MODIOLA ANODONTA UNIO NUCULA PEC- 
TUNCULUS ARCA CUCULLCEA TRIGONIA TRIDACNA 
HIPPOPUS CARDITA ISOCARDIA CARDIUM CRASSATELLA 
PAPHIA LUTRARIA MACTRA ERYCINA PETRICOLA 
DONAX TRIGONELLITES VENUS CYTHEREA VENERICARDIA 
CYCLAS LUCINA TELLINA....CAPSA SOLEN SANGUINOLARIA 
GLYCEMERIS MYA PANOPEA. 
We now proceed to the examination of the fossil remains of bivalve 
shells, the dwellings of acephalous molluscie, having no distinct head, 
and therefore unprovided with eyes, ears, &c. 
Bivalves, with equal valves. 
XCII. Pinna . A cuneiform, longitudinal bivalve, with an acute 
base, gaping in the upper part : the hinge without a tooth, lateral, 
and very long : the valves coalescent. 
One species only, P. margaritacea , Lam. is found in fragments, at 
Grignon. Specimens which may be referred, perhaps, to the same 
species, are found in the cliffs in the neighbourhood of Bognor : these 
