GIRT YELL A 271 



Brachial valve a little less convex than the pedicle, the greatest con- 

 vexity posterior to the middle, along the median line from the beak to the 

 front the surface is gently convex with the curvature a little greater 

 posteriorly, from the middle of the valve to the front the surface is 

 sometimes almost straight, transversely it is gently depressed convex 

 towards the front but becomes more convex posteriorly because of the 

 decreasing width of the valve; mesial portion of the valve somewhat 

 broadly flattened with no fold or sinus; the beak pointed, incurved 

 beneath that of the opposite valve. Internally the crural plates are 

 entirely separate from the socket plates, they diverge from the apex of 

 the valve and rise from its inner surface, the attachment extending 

 from the beak for nearly one-fourth the length of the valve ; between 

 the crural plates the concave muscle-bearing plate extends anteriorly in 

 the median line of the valve some distance beyond the attachment of the 

 crural plates and terminates in a point. 



Surface of both valves nearly smooth or marked by obscure concentric 

 lines of growth. Shell structure finely punctate. 



Remarks. This species is especially characterized by the broad, trans- 

 versely flattened pedicle valve, the absence of mesial fold or sinus in 

 both valves, the gentle longitudinal convexity of the brachial valve, and 

 the anterior position of the greatest width of the shell. It is somewhat 

 similar to D. bovidans of the Pennsylvanian faunas bxit entirely lacks the 

 sinus of the pedicle valve which is characteristic of that species. 



Locality. Washington County, Arkansas. 



Genus GIRTYELLA Weller 



Description. Shell terebratuliform, the pedicle valve sinuate, with a 

 large, subcircular or subovate, oblique foramen which encroaches upon 

 the umbo; the brachial valve frequently sinuate and often with a slight 

 median fold in the bottom of the sinus. Internally the dental lamellae 

 are well developed in the pedicle valve. In the brachial valve the socket 

 plates are joined by a concave hinge-plate which is imperforate at the 

 apex and is supported by a median septum ; the inner sides of the dental 



FIG. 31. A series of nine cross-sections of the rostral portion of the shell ot 

 Girtyella indianensis (X 2 1 /&) from the Pella beds, showing the dental lamellae 

 of the pedicle valve, and the median septum, hinge-plate, and manner of 

 formation of the crura in the brachial valve. 



