408 Dr. J. E. Gray on some Families of Bivalve Shells. 



sides tuberculated. Limbs armed with tubercular sharp-pointed 

 spines. 



About 1^ inch long. 



Hab. Adam's Peak (5600 feet). 



This novel form of Tree-frog was found by Professor Schmarda 

 of Prague in his late visit to Ceylon, in company with Chevalier 

 Fridau and Baron Konigsbrun. 



Ceylon, Galle, January 28, 1854. 



XXXYII. — A Revision of the Arrangement of the Families of 

 Bivalve Shells (Conchifera). By John Edward Gray, Ph.D., 

 F.R.S., V.P.Z.S. 



I HAVE lately had occasion to examine the animals of several 

 genera of Bivalve shells, and to consult and compare all the 

 figures and descriptions of the animals of the different genera I 

 have been able to find in the various works and essays on 

 this subject, for the purpose of preparing for the press the 

 text of the fifth and concluding volume of Mrs. Gray^s work, 

 ' Figures of MoUusca.^ With these materials before me, I was 

 led to consider, as I have done on several other occasions, the 

 characters which have been used to separate the various families, 

 and those which unite them into larger groups, — a subject sur- 

 rounded with difficulties, when we consider the very great uni- 

 formity which exists in the animals of this class, and the modi- 

 fications which habitation and modes of life produce in genera 

 which are evidently nearly related to each other, as shown in a 

 former communication (Ann. & Mag. N. H. 1853, vol. xi. p. 402). 

 After repeated comparison, and forming lists of the families 

 according to the various characters, after the example of Adanson, 

 and thus obtaining those which appear to be least variable in 

 the greater number of the families, I am induced to believe that 

 the division of the Class into Orders, according to the presence, 

 absence, and number of the siphonal openings, as proposed by 

 Poli, and followed by Cuvier, Gardner, and many other natural- 

 ists, but without paying any attention to the length or shortness, 

 the retractility or contractility of these organs, as was done 

 by those and most of the authors who have succeeded them, — 

 is decidedly the best and most natural*. 



* M. Deshayes' ' Traite elementaire de Conchyliologie,' 1843-1850. A 

 conchologist, wedded to the Lamarckian school, has asked, with considerable 

 critical acrimony, " Et d'abord pourquoi M. Gray commence-t-il la classe 

 des Conchiferes par la faraille des Venerides ? Nous avouons ne pouvoir le 

 deviner. Jusqu'ici tons les classificateurs, quelques soient les principes de 

 leurs methodes, ont toujours ete entraines par la nature des choses a placer 



