Weno and Pawpaw Formations 129 



the ventral margin. There is also a deep rounded concealed recess under 

 the umbo, from this viewpoint. The anterior adducter muscle scar is a 

 small subcircular basin-like depression with a raised rim whose dorsal 

 border connects with the pallial line, a faint narrow groove which ap- 

 proaches the ventral margin as it courses towards the post adducter scar. 

 This scar occupies a rounded-triangular elevated area lying between two 

 more elevated ridges. 



The more anterior ridge is a shelf-like lamina which separates the body 

 cavity proper from the siphonal (rostral) extension of the shell; and as 

 it passes umbonally it unites with the more posterior ridge forming a 

 U-shaped junction and enclosing between the two ridges the scar of the 

 adductor muscle. The united ridge continues anteriorly as the ventral 

 edge of a broad excavated pit or socket. The external (prismatic) layer 

 of the valve forms a distinct umbonal imbrication which makes up the 

 dorsal edge of this socket. This inner edge of the external layer passes 

 anteriorly, including the umbo but excluding the tooth against which it is 

 apposed; and proceeds to encircle and parallel the anterior and ventral 

 margins of the valve as far as the ventral border of the rostrum, making 

 an imbricated impression about midway between the pallial line and the 

 ventral margin. The thick external portion is cancellated and penetrated 

 by irregular concentric airspaces formed by the loose apposition of long 

 lamellae. The external prismatic portion makes up .8 of the thickness 

 of the shell ; the inner portion is porcellanous and in places pearly-irides- 

 cent. 



Rostrum: The inner layer emerges from the body cavity proper onto 

 the rostrum, where it is thrown into two small elevated folds which cross 

 each other almost at right angles and consequently form four depressed 

 spaces sloping away from them at either side. The shorter fold is in 

 the antero-posterior axes, and the longer one is continuous with the um- 

 bonal imbrication mentioned above. 



DISCUSSION : The description of the unfigured Cragin species C. cras- 

 sicostata, agrees with this species in some particulars, but disagrees es- 

 pecially in the number of concentric ribs. In Cragin's Kansas material 

 this number is "7 or 8 on the basal half of a shell the same number of 

 millimeters high," while in our material the number is 11 to 12. Cragin 

 states that the Kansas and the Denison specimens show "no differences of 

 specific value," but further information of a deciding nature is not forth- 

 coming. 



Cragin's description of the species, Corbula crassicostata, is as follows: 1 



Cragin, Colo. Coll. Stud., 5, 1895, p. 61. 



