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
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