DIELASMA 263 



pressed towards the front, sometimes produced anteriorly in a short, 

 rounded, lingual extension ; beak prominent, strongly incurved, pierced 

 by a large foramen which encroaches wholly upon the umbonal region, 

 being in contact with the delthyrium only at its apex ; delthyrium broadly 

 triangular, hidden by the incurvature of the beak;. Internally the dental 

 lamella? are well developed and of moderate length, the muscular scars 

 obscure or obsolete. 



Brachial valve less convex than the pedicle, the greatest convexity pos- 

 terior to the middle, the surface arched from beak to front with the 

 curvature becoming continuously more convex towards the beak, the 

 lateral slopes convex, curving abruptly from the median line to the lateral 

 margins; mesial portion of the valve not differentiated as a fold to cor- 

 respond with the sinus of the opposite valve, but the anterior margin 

 commonly exhibits a rounded sinuosity to correspond with the short lin- 

 gual extension of the opposite valve ; beak pointed and incurved beneath 

 that of the opposite valve. Internally the crural plates are separated from 

 the socket plates and diverge anteriorly, at the beak they are low, 

 angular ridges but become thin, rather highly elevated lamellas an- 

 teriorly, they are joined transversely by a concave plate bearing the 

 muscular scars which is attached to the inner surface of the valve along its 

 median line, leaving a pair of triangular, finger-like cavities converging 

 towards the beak, which are surrounded by the inner surfaces of the base 

 of the crural plates, the muscle plate and the inner surface of the valve ; 

 anteriorly the muscle-bearing plate extends considerably beyond the ter- 

 mination of the attached portion of the crural lamellas ; the muscle scars 

 are commonly rather strongly impressed upon the surface of the concave 

 muscle plate. 



Surface of both valves smooth externally except for the presence of con- 

 centric lines of growth which vary greatly in strength upon different 

 individuals; upon some internal casts exceedingly faint radiating costae 

 are sometimes distinguishable towards the lateral and anterior margins. 



Remarks. This species has been identified sometimes as Girtyella turgida, 

 but besides being a much larger species and lacking the mesial depression 

 of the brachial valve towards the front, the internal structure of the 

 rostral portion of the brachial valve is totally different and the two species 

 cannot even be included in the same genus. The brachial valve of the 

 species has been identified sometimes as D. formosa, but it differs from 

 the corresponding valve of that species in being somewhat flatter medi- 

 ally, in having a less pointed anterior outline, and in having the mesial 

 sinuosity in the anterior margin for the accommodation of the anterior 

 extremity of the sinus of the opposite valve. 



Horizon. Keokuk limestone, Salem limestone. 



