Dr. J. E. Gray on some Families of Bivalve Shells. 33 



by Recluz, the operculum resembles a small pulley, instead of 

 being cup-shaped as in the more typical species. 

 Malvern, November 29, 1852. 



Note. — In a copy of Pfeiffer's ' Monographia Pneunopo- 

 morum' just received, I find an amended description of his 

 Cyclotus Taylorianus (Zeitsehr. 1851), to which, in a subsequent 

 note (p. 50), he assigns C. Charhonnieri as a synonym, and re- 

 marks that Pterocyclos biciliatus, Mousson, is closely allied to it, 

 if not identical. A comparison with the specimen at Ziirich will 

 decide. If identical, the name Taylorianus must give way to 

 Mousson's designation. The structure of the shell is that of a 

 Pterocyclos. The operculum shows it to be an aberrant species, 

 but does not quite conform to that of Cyclotus. — W. H. B. 



December 22, 1852. 



V. — A Revision of the Genera of some of the Families of Con- 

 chifera or Bivalve Shells. By J. E. Gray, Ph.D., F.R.S., 

 V.P.Z.S. &c. 



Several of the families of Bivalve Mollusca are well circum- 

 scribed, and the genera of other families are well defined, but 

 one of the problems of systematic malacology is the arrangement 

 of the families into groups and into a natui-al series. Each cha- 

 racter which has in succession been chosen, and, indeed, each 

 group of characters which has hitherto been studied and used 

 for this purpose, appears to fail when an extensive series of the 

 animals and their shells have come under examination for the 

 purpose of verifying the system proposed. Under these circum- 

 stances, I have thought it desirable to turn my attention to the 

 examination of the smaller groups or families, and to attempt to 

 divide them into natural sections and genera, until some fortu- 

 nate combination of circumstances should show the systematic 

 zoologist how the families can be placed in a more natural series 

 than the provisional one now adopted. Following out this idea, I 

 have lately, at various times, studied the species of certain fami- 

 lies of bivalve shells which appear most to require revision, con- 

 sidering this the more necessary as these shells have hitherto 

 been divided in a most unequal manner. Some genera, as 

 Cardium, Mactra, Tellina, &c., ai'e magazines, containing very 

 many kinds ; while many other genera of bivalve shells have 

 been established on a single species, having some slight modi- 

 fication in its cardinal teeth, or some anomalous external form, 

 which, when compared with other species of the family, is not 

 of so much importance as the peculiarities in the shells ofifered 

 by many kinds which have been left as species in these large 

 Ann. &; Mag. N. Hist. Ser. 3. Vol. xi. 3 



