NO.H20. rROCEEDiyaS of the XATIOyAL MUSEUM. 717 



cardinal slope forms a narrow, rounded, obscure false area on each side 

 of a small, low, slightly convex pseudodeltidium. Both tlic area and the 

 pseudodeltidium are scarcely observable in most specimens. On casts 

 of the intei ior the pseudodeltidium is more i)lainly shown (Fig-. 46;), but 

 the general surface of the exterior of the shell rounds into the false 

 area, with scarcely any line of demarcation. A section of the cast of 

 the ventral valve indicates that it was thin at the margin, increasing 

 in thickness toward the apex, where it was as thick in proportion as in 

 the ventral valve of the Yorlia iraimeri. The cast Fig. 4c shows that 

 on the interior of the shell there is the opening of a large foramen 

 which tapers to a small apex, and that Just in advance of the foramen a 

 short median depression occurs, on each side of which a slight elon- 

 gated tubercle arose. Outside of the tubercle a small, elongated, 

 depressed muscle scar is seen, which was separated irom a deep, 

 brrongly marked vascular sinus by a very narrow, sharp ridge. The 

 vascular trunks start on the sides of the pseudodeltidium, just back of 

 the foraminal opening, curving around it to the central muscle scars 

 and then extending forward to the center of the shell. Small, round, 

 lateral scars appear to be indicated back of a sharp ridge that bounds 

 the posterolateral margin of the vascular trunks. 



Dorsal (brachial) valve with a nearly transverse hinge line that is 

 considerably narrower than the greatest width of the shell. The car- 

 dinal slope forms a rather narrow area, which is broken midway by a 

 low, slightly convex pseudodeltidium. The cast of the interior shows 

 that there was present in the interior of the shell a median ridge that 

 extended from under the beak forward beyond the center of the shell 5 

 on each side of the median ridge, beneath the umbonal ])ortion of the 

 shell, a hollow occurs between the ridge and what apjiears to be a 

 slightly depressed path of advance of a muscle scar. The features 

 described occur upon a siliceous cast preserving the natural convexity 

 of the shell. In a compressed specimen in the shales in which the 

 siliceous nodules occur, the median ridge is well shown, and extends 

 to the center of the shell, where a pair of small anterior muscle scars 

 rest against it. In front of the scars the ridge sends off a number of 

 tine lines or striie toward the front of the shell; the antero-lateral scars 

 occur on each side of the ridge, about one-half the distani-e from the 

 beak to the front margin of the shell. The posterolateral scars are 

 also indicated toward the posterolateral margin. 



Surface of both valves marked by very tine concentric stri;e and 

 extremely tine radiating lines of growth. Substance of shell corneous. 

 It is not ])reserved in the siliceous nodules, but is shown as a black, 

 almost phosphatic shell in the shales. 



Formation and locality. — Middle Cambrian, Coosa shales. Cowans 

 Creek, Cherokee County, Alabama. (No. 2<!J:30, U.S.N.M.) 



