434 Dr. J. E. Gray on the genus Assiuiinia. 



to form more correct views as to the affinities which exist between 

 the different species of the kirger and smaller groups, and thus 

 obtain a more clear insight into the relations which the animals 

 bear to each other and to the rest of the creation. 



In proportion as the number of animals known to naturalists 

 has increased, by the extension of the study, the more careful 

 researches of collectors, and the more minute examinations into 

 the structure and habits of the different kinds, the number of 

 groups into which they have been divided (whether called genera 

 or subgenera) have been and are continually increasing also. 

 For if the number of species in a group is inconveniently large, 

 the object in forming the group has been overlooked ; hence a 

 group containing 600 or 700 species is found to be of compara- 

 tively little use in Natural History, and I must consider that a 

 naturalist who proposes to I'cduce well-established groups, and 

 to refer the species to such large groups, is evidently retrograding 

 instead of advancing scientific objects. Such considerations 

 sufficiently prove that it is an advantage " that a genus should 

 be restricted in the number of its species * ;" and there cannot 

 be a doubt " that such notions are held (almost universally) by 

 modern zoologists,^^ although Mr. Clark may not have been 

 " before aware " of it. 



Every day's experience confirms me in the opinion which I 

 have often expressed, that in the distinction of the larger and 

 smaller groups of Mollusca the characters derived fi-om the ani- 

 mal, the shell and the operculum, which all have a mutual rela- 

 tion to each other, are of equal value and constancy, care being 

 taken to select such parts of them as depend on organic struc- 

 ture and are not liable to accidental variation. This care in 

 selection is equally necessary in relation to the animal, as it is 

 to the shell and operculum. Such was the idea of Lister and 

 Adanson ; but while some authors, like Lamarck, profess to 

 arrange the Mollusca according to the supposed structure of the 

 animal (but in fact took all their characters from the shell, as 

 only so few animals wxre known in their time), — others, as 

 Ferussac, have declared that the form of the shell should be dis- 

 regarded, and that no genus is good that is not founded on the 

 structure of the animal. By the remark of Dr. Philippi, ap- 

 pended to the part quoted by Mr. Clark, he appears to have 

 been of that opinion when he wrote the paragraph, though I 

 believe he has since modified it ; and Mr. Clark, in some of his 



* I am aware there are some genera, as Conus in Malacology and Sola- 

 num in Botany, which contain very many species; but this "arises from 

 permanent characters not having yet been found by rthich they may be 

 divided, and not from any disinclination on the part of naturalists to divide 

 them. 



