BIVALVES. 



PLATE LX. 



Genus. ARC A. 



Character. Shell, valves of an equal size, having a square elbow at one end, 



and round at the other; hinge having numerous teeth sharpened, and placed 



alternately. 



Species. 



No. 1. Arca elegans. Shell of a pale red, white and blue within, the outer edge 

 undulated. A native of Sierra Leone and the Gold Coast. From a shell 

 in the Author's Museum. 



No. 2. Arca rubra. Shell of a strong red colour, streaked with red veins, the 

 inside of a pale gray ; the two shells being delineated together, convey to 

 the mind an idea of Noah's Ark, being formed like a ship in the lower 

 part, and covered over at the top, from which circumstance it is supposed 

 this shell received its name. 



Genus. OSTREA. 

 Character. Shell, externally hollowed out, imbricated, and rugged ; the valves 

 unequal in size, hollowed out in the manner of an ear. The hinge having teeth 

 slightly marked. 



No. 1. Ostrea dentata. Shell brown, lined with white and blue shades; the 

 upper shell small, and of a flesh colour; this shell differs in form, in many 

 respects, from the common oyster, particularly in its branched sides. It is 

 a native of the East Indies. 



Genus. ANOMIA. 

 Character. Shell, the valves unequal in size, one more rounded in the base than 

 the other ; sometimes one of the valves has a round hole near the hinge ; hinge 

 having only a lineal channel or ridge, but which is confined to that part. 

 No. 1. Anomia ovata. Shell of a pale brown, oval, having a circular opening 

 near the hinge; by passing a muscular membrane through this hole, the 

 animal is supposed to attach itself to the rocks at the bottom of the sea. 

 No. 2. Anomia undulata. Shell of a sinuated form, and gibbous below. A 

 native (as well as the foregoing) of the Seas of the East Indies. 



REMARKS. 



The genus Arca is well worthy the accurate examination of the skilful Conchologist, from 

 its very singular construction, the teeth of the hinge slanting different ways ; it seems to 

 have a distant analogy to the genus Pecten, from which, however, its oblong and smoother 

 form will always distinguish it. The Ostrea, or Oyster, is too well known to need any 

 particular explanation. The Anomia is deserving of very particular attention, on account 

 of Linnaeus having supposed that fossil shells of this genus were to be found in shape and 

 character exactly agreeing with the recent ones, to which he has given the same name. This 

 was, however, one of the accidental errors of that great Naturalist, for, although, they may 

 agree in the generic character, yet in their exact specific shape, a constant difference occurs. 

 In the examination of forty-five different fossil Anomiae found in Derbyshire and other parts 

 of England, I did not find one of them to agree perfectly in shape with any living species. 



