INSTITUTE OF SCIENCE, PHILADELPHIA. 495 



dont. If it be considered desirable to name this type of tooth, perhaps Neu- 

 inayr's term Dysodont may be restricted for the purpose, though his original 

 use of it was coextensive with the Anisomyaria. When the element of torsion 

 is absent the same stock might give rise to forms like Vamtxemia and Macro- 

 don, undeveloped or abberrant Taxodonta. 



Of the Teleodesmacea in the most perfected forms, the tendency was to 

 form a hinge-plate and in all to separate the guiding, or cardinal, teeth from 

 the fixative, or lateral, laminae. When the cardinals became solid and strong 

 and other features rendered the fixative function of the laterals less important, 

 the latter frequently became obsolete or feeble. Among the early Teleodonts 

 the hinge-plate is less conspicuous and, no doubt, as in other groups, was orig- 

 inally absent. In some forms, in which the thin shell was subjected to con- 

 siderable torsion, a type of tooth was evolved which has persisted in a few in- 

 stances to the present day, and is sufficiently distinct from the ordinary type to 

 deserve a name. This is that flat and twisted form of armature exhibited by 

 Isocardia, and to a less degree by the various forms of Cardium, where the 

 teeth seem to spring from the cavity of the beaks and to be, as it were, twisted 

 into line on the cardinal margin. For these I would'propose the term Cyclodunt. 



A sharp distinction must be drawn between the transverse striation verg- 

 ing into denticulation which is exhibited by Pcctcn and Spondyliis, and which 

 represents the obsolescent ancestral taxodont dentition, on the one hand, and 

 the transverse striation observable on so many Teleodont teeth as well as with 

 those of some of the specialized Prionodonts.* Crassatellites, Mactru, Unio, Cas- 

 talia, Trigonia and others often show such striations, sometimes very sharply 

 developed. But these are simply the result of transverse friction acting on the 

 proliferations of the mantle between the teeth, induced by the to-and-fro motion 

 of the valves and preserved or emphasized by natural selection as an additional 

 protection against irregularity in opening and closing the valves. These 

 stria: simply aid in guiding the valves and may or may not be developed in 

 closely related species of the same genus. Close study will show that the 

 striations are invariably in the line of motion and parallel to it. It is conceiva- 

 ble that functional teeth might be developed in this way, but only when they 

 were preceded by other teeth upon whose surfaces they might be initiated. 

 True teeth, on the other hand, owe their initiation to the influence of external 

 sculpture upon the hinge-margin and therefore are morphologically different. 



In the Anomalodesmacea we have a tribe of burrowers which have pre- 

 served to the present day (in spite of much specialization) some of the features 

 which characterized the edentulous Protopelecypods of ancient geological time. 

 The nearly edentulous hinge, the more or less nacreous or earthy texture of 

 the shell, the frequent asymmetry and the subequal adductors high in the 

 shell are all archaic features, and we may suppose that their preservation is 



Thi distinction was overlooked In my early references to the subject. 



