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



others have placed the Unioniform shells in the totally distinct 

 genus Cardiaia, and the Modioliform species in that of Myalina. 



As it is possible that I may have to notice the last-named 

 genus on a future occasion^ I purpose confining myself, in the 

 present paper, to the Unioniform group. 



Some yQuvs back, when residing in Newcastle-on-Tyne, I 

 occasionally obtained some good casts and testiferous specimens 

 of the so-called fossil Unios from Jarrow pit and some others in 

 that district ; also from a clay-ironstone band near Whitley. 



On examining these specimens, I ascertained that they pos- 

 sessed certain characters sufficiently distinctive, and were deficient 

 in a very important one, to warrant their removal from the genus 

 in which they were usually placed. This led me to propose, in 

 the Annals for November 1844*, a new genus for the fossils in 

 question, under the name Anthracosia, on the occasion of my 

 nominating and describing some other fossil genera, — intending 

 to publish the diagnosis of it shortly afterwards. I need not 

 enter into any explanation of the causes which have prevented 

 this intention being carried out; suffice it to say, that, as several 

 parties have adopted the name I proposed, and as no one, that 

 I am aware of, has yet published any generic description of these 

 fossils, I feel myself bound to take up the subject, even after the 

 lapse of so many years. 



The character which Anthracosia is deficient in, exists in all 

 the known genera of Unionidce. It consists of a largish scar in 

 each valve, situated immediately behind the impression of the 

 anterior adductor muscle, and which is evidently supplementary 

 to the usual number of retractor muscles belonging to the foot 

 of other Dimyarian shells, and inserted within or near their um- 

 bonal cavities. To distinguish the scars left by the supplementary 

 muscles from those produced by the ordinary anterior foot-retrac- 

 tors, they have sometimes been termed " supplementary pedal 

 muscular scars. ^' 



Reverting to the Unioniform fossils, several fine casts have 

 passed under my notice; but in none have 1 observed the 

 '^supplementary scars," — only those usually situated, as in 

 ordinary bivalves, v/ithin or near the umbonal cavities. The 

 remarkably fine cast, represented in PI. IV. fig. 5, is exceedingly 

 instructive in this respect. 



The absence of these scars appears to have given rise to the 

 opinion, that the fossils under consideration, though admitted 

 to be freshwater species, belong to a marine genus, or, at least, 

 to one {Cardiaia) originally formed for some well-characterized 

 marine shells. 



* " On a new Genus of Palneozoic Shells." — Annals and Magazine of 

 Natural History, vol. xiv. 1844. 



