2 CARBONIFEROUS LAMELLIBRANCHIATA. 



aphorism tliat the larger the number of specimens examined the fewer the species 

 determined, or that the number of species varies inversely with the number of' 

 specimens studied. 



The question of species, and to some extent that of genera, is largely a personal 

 one. Each individual observer gives a different value to variations in shape, size, 

 or even to small anatomical differences ; and when, as in palseontological research, 

 it is impossible to obtain any but the most elementary evidence of the anatomy of 

 the animal itself, and one organ — the test — has to furnish the characters for classi- 

 fication, the most reliable portions, the hinge and interior, often being invisible 

 (too-ether with the imperfections of the geological record), an approximation only 

 can be made to accuracy in zoological determinations. 



The work of American authors is scattered through numerous publications 

 access to which is often difficult. In many cases no figures accompany the 

 descriptions, and the type specimens are not recorded, and are scattered over 

 that Continent in private collections. Very few British species are recognised as 

 occurring in American Carboniferous rocks, though there is a very close resem- 

 blance between the two faunas, and but small differences seem to exist in many 

 cases between British and American species — an extent of variation not greater 

 than would be expected to exist in a single species surviving in such widely 

 separated areas. The greater accuracy in generic delineation is greatly due to the 

 recognition by American authors of the importance of the internal characters of 

 the shell, and to the fine state of preservation in which in many cases specimens 

 occur in American beds. Many of their generic names have been accepted by 

 de Koninck and others, in order to mark off recent from palgeozoic fossil genera. 



Fortunately in many cases the original types of the published British Carbo- 

 niferous Lamellibranchs are still extant. The " Ure Collection " is preserved in 

 the Museum of the Royal Society of Edinburgh. 



The few forms described and figured by Sowerby in the 'Mineral Conchology ' 

 are, the writer believes, all present in the Sowerby Collection of the Natural History 

 Museum at South Kensington, and in the same place are those specimens which 

 were described by Phillips in the ' Geology of Yorkshire,' from the Gilbertson 

 Collection, while Portlock's specimens are all safe in the Museum of the Greological 

 Survey at Jermyn Street. The Griffiths Collection, the basis of M'Coy's first 

 great work, is preserved in the Science and Art Museum, Dublin ; but many of the 

 specimens figured, probably those from private collections, have been lost sight 

 of. The specimens figured in M'Coy's second great work are all in the Wood- 

 wardian Museum at Cambridge, while those described by the palaeontologists of 

 the Geological Survey are in the museums of that Department in London, Edin- 

 burgh, and Dublin. By the courtesy of the custodians I am permitted to 

 refigure in every case the original specimens. 



