Report of tbi Geologist. 595 



each <>f these scars is smoother than the Uy much 



thickened and often extremely I at its outer margins. 



These may be regarded as accessor} elements of the posterior 



adductors. b An anterior pair anterior adductors), situated 

 close together at about the center of the yalve. The position of 

 these is generally well defined but their outline is frequently 

 obscure, [c) An elongate, narrow median scar, which is appar- 

 ently divided for its entire Length by a faint ridgg In front of 

 the muscular area there are of ten a Dumber of short protuberances 

 on each side of the median septum and the anterior pair of scars 

 is frequently obliterated by prominent callosities. At the line 

 of geniculation the interior surface is elevated into a very promi- 

 nent, sharp, or abruptly rounded crest. Spiral callosities for the 

 support of the brachia, similar to those in Dayidsonia and 

 Lfpt.enisca, have been observed. 



Shell-substance strongly punctate. 



Type, Lepkena rugosa, Dalman. 



Distribution. Silurian — Carboniferous. 



Rafinesquina, Hall. J 892. 

 (Plate 13, Figs. 513-18.) 



Shells normally concavo-convex. Surface ornamented by radia- 

 ting stria?, of alternating size, crossed and crenulated by finer 

 concentric stria?. Cardinal margins without denticulations. 

 Interior of the pedicle-valve with the muscular area not strongly 

 limited; consisting of two broad flabellate diductor scars inclos- 

 ing an elongate, more distinctly defined adductor. The faintness 

 of the limitation of this area is in marked contrast to the sharply 

 defined muscular area in the corresponding valve of Li'llva. In 

 the brachial valve the cardinal process is more closely sessile 

 than in Lepj .kna, and there is frequently a linear callosity between 

 the branches. The posterior adductor scars have the arborescent 

 markings of Leptmna rhorriboidalis^ and these impressions are the 

 only ones well defined, the anterior scars being narrow and rarely 

 retained with distinctness. From the anterior margin of the 

 muscular area radiates a series of irregular furrows and nodose 

 ridges, which are to some extent of vascular origin. 



Type, Rajvtiesquina alternata, Conrad (sp.). 



Distribution. Lower — (?) Upper Silurian. 



147 



