The Hinge of certain Non-Marine Lamellibranchs from the Lenisulcata Zone of the Coal Measures

Abstract

(1) The Carbonicola fallax group of shells, which characterize the Lenisulcata Zone of the Coal Measures, is an extremely variable one comprising a wide range of forms, between which there appears to be perfect gradation.(2) The hinge of the group is also a highly variable feature, bearing little relation to the outline of the shells. Its variation, however, is more fully known in small forms than in large ones.(3) Evidence submitted in the present paper indicates that, although the hinge pattern of the C. aquilina group may not be as constant a feature as has hitherto been supposed, the group is distinct from that of C. fallax. There is no evidence suggesting genetic relationship. A basis for the distinction between internal moulds of C. aquilina and C. fallax is fully described.(4) The basic features of the hinge of the C. fallax group are essentially those which characterize the Carbonicola pseudorobusta–C. communis group of the Ovalis Zone. While certain of the hinges of the larger shells appear typical of the latter group there are a number of other patterns which have not previously been described in the Carbonicolidae.(5) The hinge features of the C. fallax group appear less specialized than those of shells from the Ovalis Zone, especially Carbonicola pseudorobusta Trueman (MacLennan, 1944).(a) Large strong teeth have not been seen. Definite small teeth are not commonly developed. It is usual to find gentle swellings on the hinge plate corresponding to depressions in the opposing valve. Some forms are quite edentulous.(b) Only one tooth has been found in either valve, although more than one swelling or depression may be present. There is no approach to the more complicated patterns figured by MacLennan (1944).(c) In most cases engagement of the dental apparatus would only take place as the valves opened. Notches or sockets in the hinge plate are not necessarily opposed by teeth or swellings, but they would have been more or less filled as the opening continued, when the hinge plates apparently rolled against one another.(d) In a community of small shells there is evidence to suggest that in several cases the hinge margin of the left valve slightly overlapped that of the right for a short distance anterior to the umbo.(6) The range of variation of hinge characters in a community is very large. Collections from different horizons, however, may indicate individual peculiarities or tendencies in the variation. Whether these owe their origin to local conditions or evolution is not yet clear.(7) The hinge of Anthracomya from the Lenisulcata Zone is as yet little known. What evidence there is points to a hinge similar to that of small Carbonicola showing the same type of variation.