Prof. W. King on the Fossil genus Anthracosia. 53 



But, notwithstanding the absence in Anthracosia of the sup- 

 plementary scars peculiar to the Unionidce, there can be little 

 doubt, from the presence of another essential feature, of the genus 

 belonging to the family just named. 



In most Dimyarians, the ligament extends from the corselet 

 (where it covers the cartilage, when this last part is external) to 

 a little in front of the umbones; but the umbonal portion is 

 seldom well developed, except in certain species — notably in Cy- 

 prina Islandica, in each valve of which it occupies a rounded cavity, 

 excavated in the hinge-plate immediately beneath the umbone, 

 and in front of the cartilage. In the tfnionidcB, however, the 

 umbonal portion is as much developed as the corselet division ; 

 and that part of the hinge-plate, on which it is situated, is never 

 separated from the teeth by so marked a line of demarcation as 

 prevails in marine shells. These peculiarities may be distinctly 

 observed in our British Unios : they are also to be seen in Unio 

 littoralis, and more or less in all the American species. In these, 

 as well as marine shells, the corselet ligament is more compact 

 than the umbonal portion, — the latter being composed of imper- 

 fectly conjoined laminae, which, in Unionidce, produce a number 

 of parallel curving linear impressions, commencing from above 

 the cardinal teeth, and descending behind them, nearly to the 

 inner or free margin of the hinge-plate. When the umbonal 

 portion of the ligament is removed, impressions of its constituent 

 laminae are quite obvious on the hinge-plate. 



The fossil genus Anthracosia has evidently possessed the liga- 

 mental peculiarities of the Unionida in an eminent degree, as 

 that part of the hinge-plate on which the umbonal ligament was 

 implanted is comparatively large and deeply excavated; — indeed, 

 in the typical species it is much larger and deeper than in any 

 existing forms that have passed under my notice. 



I repeat then, there can be little doubt that its ligamental 

 characters prove the genus Anthracosia to be a member of the 

 family UnionidcB-, but as its mollusk was not furnished with the 

 usual supplementary pedal muscles, it may be considered as 

 representing an abnormal section. 



Family UNiONiDiE. 



Genus Anthkacosia*, nobis. 



Syn. Unio, Sowerby and others. 



Pachyodon, Brown (not Stutchbury), 

 Cardinia (in part), Morris and others. 



Diagnosis. — Equivalved : inequilateral. Teeth — one in each 



* From avOpa^ = carbon, in allusion to the carbonaceous deposits in 

 which the genus is usually found. 



