3 4 Dr. J. E. Gi'ay on some Families of Bivalve Shells. 



genera, or is of less importance, when wc consider the change 

 in the organization of the animal, which must produce the cha- 

 racter selected for the purpose of separating them from their 

 allies ; — differences which are constantly overlooked in the study 

 of the species of the larger genera. 



Hitherto modern conchologists seem, on principle, to have 

 avoided the examination, revision, and analysis of the genera of 

 these shells ; as I cannot call to mind a single author, either in 

 England or abroad, who has within the last ten or twelve years 

 published any paper on a single family of bivalve shells except 

 myself; indeed they remain nearly in the state they were left 

 by Lamarck, except as far as regards the description of certain 

 isolated genera formed for a few and often a single newly disco- 

 vei-ed species. 



This apparent neglect of the subject has most probably arisen 

 from the difficulty of studying the species of the genera in detail, 

 witiiout having the facility of examining a large number of the 

 specimens of each species in their various states of growth at the 

 same time, and of moving them about, so as to see how the spe- 

 cies of the family or genus agree with or differ from each other 

 at a single view, — a kind of examination which the small drawers 

 of the cabinet generally used do not well afford. The British 

 Museum collection, where all the specimens of the different spe- 

 cies are attached and arranged in the same position on moveable 

 boards, affords me, in common with any other conchologist who 

 is willing to study it, greater facilities for this kind of comparison 

 than any other collection I have seen either in this country or 

 on the continent. It is probably the facilities which this col- 

 lection has afforded me for studying the affinities of the genera and 

 families of shells, that have induced so many of the most scientific 

 conchologists to receive with such kindness the observations on 

 the genera and families of shells published in the * Synopsis * 

 of the British I\Iuseum for 1840 and 1842, and my papers on 

 the genera of Venerida, Mactrada, Anomiada, Placuniada, Pho- 

 ladida, &c., which have been published in the various journals. 



It is this attention, and the hope of forwarding the study of 

 a very favourite part of malacology, which have induced me to 

 send the following synoptical revision of the genera of certain 

 families of Bivalves for insertion in the ' Annals,' premising that 

 in the Catalogue of the Bivalve MoUusca now in progress, the 

 characters of the genera will be given in greater detail. 



I may here observe, that I regard the shoi'tness of the character 

 as an advantage, enabling the student easily to identify the group 

 of genera and the genus to which any particular species under 

 examination belongs. It is the custom of many zoologists to 

 give extended characters of the genera and long descriptions of 



