54 Prof. W. King on the Fossil genus Anthracosia. 



valve below the umboue, rather low and massive : crown of tooth 

 of right valve excavated anteriorly and ridged posteriorly: crown 

 of tooth of left valve ridged anteriorly and sloped posteriorly. 

 Umbonal ligamental fulcra, each a furrow excavated in the 

 hinge-plate, between the umbone and tooth. Scars of the ante- 

 rior set of pedal muscles situated above the anterior adductor 

 muscular impressions. 



Typical species, Unio Beaniana^ . 



The dental system of Anthracosia is much simpler than that 

 of most Unionidce-j and it differs so much from what usually pre- 

 vails in the family as to appear formed on a totally different plan. 

 The dental formula is merely — Cardinals ~ ; and the interlocking 

 of the teeth is simply thus — the excavation of the right-valved 

 tooth receives the ridge of the left-valvcd one; and the ridge of 

 the former fits on to the slope of the latter, as is exhibited in the 

 diagram section represented under fig. 3, PI. IV. In other den- 

 tiferous Unionidce, however much the cardinal teeth may vary in 

 form and direction, they all appear to be formed on one plan ; 

 and their formula is thus: — Cardinals Right t" • ""'ith or without 

 Laterals, the left-valved two clasping the right-valved one (vide 

 PI, lY. fig. 4). Hence in the latter shells the teeth interlock each 

 other completely, and much more securely than in Anthracosia. 

 In the latter genus, however, this defect was undoubtedly com- 

 pensated by the unusual size of the umbonal portion of the liga- 

 ment. iS'oue of the testiferous specimens which I have examined, 

 of the type of Anthracosia, exhibit the least appearance of pos- 

 terior teeth ; nor do casts of some other species before me afford 

 any indication of their presence in the genus. It will thus be 

 obvious that the genus Cardinia, which possesses well-developed 

 posterior teeth, and which is furnished with some other differ- 

 ential characters, is not the proper group, as some appear to 

 think, for the shells under consideration. 



The fulcra of the iimbonal ligament of Anthracosia , although 

 simply an enlargement of a peculiar feature of the Unionidce, 

 nevertheless form a well-marked character in the diagnosis of the 

 genus. In the typical species they are widish furrows, broadest 

 and deepest on the inner or free margin of the hinge-plate : that 

 of the left valve is more deeply excavated than the right-valved 

 one. The linear impressions of the lamina? forming the um- 

 bonal ligament are rather prominently marked on both fulcra, 

 and they curve suddenly down (forming a deep sinus) to the 

 inner margin of the hinge-plate [vide PI. IV. fig. 2). 



Judging from the species represented under fig. 6, PI. IV., the 

 cartilage appears to have been of the usual size. Several speci- 



* Vide Sii])])lenientan NotP. 



