Prof. W. King on the Fossil genus Anthracosia. 55 



mens have occurred to me in which this structure is equally well 

 preserved; and I perceive that Captain Brown represents it in 

 his Pachyodon [Anthracosia] rugosus^. 



The impressions of both adductor muscles are usually well 

 displayed on casts. The anterior one^ which is by far the most 

 distinct, is strongly jagged in most species. The pallial line, 

 exhibiting the simple character prevailing generally in the family, 

 is also often very well marked. The scars produced by the an- 

 terior pedal muscles are limited to the umbonal cavity and the 

 edge of the hinge-plate, as in Dimyarians generally [vide PL IV. 

 fig. 5). I have searched repeatedly and carefully on casts of 

 various species for the supplementary scars, without however 

 observing any that I could safely conclude were such. Occa- 

 sionally I have seen marks on or near the part where they usually 

 occur ; but I feel perfectly satisfied that they can only be con- 

 sidered as accidental, having never seen them on good sharp casts. 

 It is necessary to mention this, as some persons might look upon 

 such marks as having been produced by the supplementary re- 

 tractors of the foot. 



Probably the absence in Anthracosia of a character hitherto 

 never found to be absent in the Unionid^e, might give rise to the 

 opinion that the genus ought to be placed in another family. 

 Further discoveries may render such a step necessary. I feel 

 persuaded, however, that whatever may hereafter be discovered, 

 the relationship hetween Anthracosia and the Unionida, contended 

 for in this paper, will not be in the least weakened. At present 

 I am only disposed to go the length of regarding it as the type 

 of an abnornal section of the family. 



Anthracosia appears to be confined to deposits of the Carboni- 

 ferous system; for all the Unionidce hitherto described as occurring 

 in secondary and tertiary deposits are normal forms of the family. 

 The Eocene Unio Solandri, and a small Neocomian species before 

 me, exhibit the supplementary scars. Mantell detected them iu 

 his Unio Valdensis; and they are well displayed on a Jurassic 

 species found in the Gristhorp plant-beds, and described some 

 years ago by my friend Mr. Bean, under the name of Unio dis- 

 tortus. This specimen appeared to me so interesting as regards 

 the character under consideration, that Mr. Bean kindly allowed 

 me to make the sketch which is given in PI. IV. fig. 7. 



Although none of the Carboniferous species that I have seen 

 display the supplementary scars, it must not be considered that 

 I have no belief in the existence of normal Unionidoi during the 

 primary period. The highly interesting form named Anodon 



* " Description of some new species of the genus Pachyodon.^' — Ann. 

 & Mag. of Nat. Hist. vol. xii. p. 391, PI. 15. figs. 6 Si 7 (1843). 



