CHAMA. 



selves by one valve, and in others to fix themselves by the 

 other valve. The Chama daniicornis belongs to the first, 

 and the C. Arcinella to the second. This last species is 

 also remarkable for being nearly equivalve, and for the 

 regularity of its spinous processes, as well as for the small- 

 ness of the attached portion and the proximity of its 

 umbones, for which reason it shows but indistinctly the 

 divaricated ligament. There is one strong, irregular, 

 oblique, thick, striated, generally crenate, hinge tooth in 

 one valve, which is received into an irregular, also striated, 

 groove in the other valve. There are two distant, lateral, 

 muscular impressions in each valve, and the line to which 

 the mantle is attached is entire. The ligament is external, 

 divided into two segments at its posterior extremity, one 

 of which is decurrent to the point of the umbo in each 

 valve. 



There are several recent species of this Genus, one of 

 which, the Chama Lazarus, is extremely variable; indeed, 

 the general resemblance of one species to another, and the 

 numerous varieties of each, renders it extremely difficult 

 to determine the species. Of fossil species there are many, 

 some of which vie with the recent in the singularity of their 

 external spinous foliations, though not in the beauty of 

 their colouring : several are described and figured by La- 

 marck in his account of the Fossil Shells of the neighbour- 

 hood of Paris, as well as in " Sowerby's Mineral Concho- 

 logy.'* They are found in the London Clay, and Calcaire 

 Grossiere also in the Chalk and Green sand. 



On account of the similarity between this Genus and 

 Diceras, we shall be expected to explain the characters by 

 which this latter is distinguished from Chama, with which, 

 indeed, it is arranged by Bruguiere ; these, according to 

 Lamarck, are the large, conical, diverging, spiral umbones, 

 and the large, concave, subauriculate, prominent tooth in 

 the large valve of the Diceras. Not having ourselves seen 

 the hinge of Diceras, we will not venture to offer an opi- 

 nion ; but judging from the specimens we possess, we see 

 in Diceras a sort of connecting link between Isocardia and 

 Chama, having both the umbones free and involute, and 

 being moreover a nearly equivalve shell, like Isocardia ;. 

 but being attached by one valve, and not quite equivalve ; 

 in these respects resembling Chama. As the Diceras is 

 only known in a fossil state, it is impossible to ascertain the 

 comparative anatomical characters of the animal. 



