92 University of Texas Bulletin 



Small end, width 4.0 4.0 4.0 3.1 



Large end, height 4.5 5.5 5.3 4.0 



Large end, width 4.3 4.8 4.7 3.6 



HORIZON: Basal two-thirds of the Pawpaw formation, clay phase. 



LOCALITY : 723 (type locality) ; 714, and other localities near Fort 

 Worth, Texas. 



DESCRIPTION: Shell discoidal, volutions slightly embracing, each 

 covering one-fourth or less of the next inner volution, umbilicus conse- 

 quently broad, volutions strongly ribbed with widely spaced, coarse, short, 

 simple or bifurcated, straight ribs, each with an umbilical tubercle and a 

 pair of coarse marginal tubercles which are connected by a concave topped 

 ridge. These tubercles are abruptly raised above the level of the venter, 

 but the oblique rapidly diminishing rib passes from the tubercle towards 

 the keel and becomes obsolete; the keel is therefore bounded on each side 

 by a narrow valley. Keel prominent, steep sided, triangular in section, 

 taller than the outer ventro-lateral tubercle in the earlier volutions, lower 

 in the later ones. Cross-section of the volutions roughly square, but in 

 a few individuals even broader 1 than tall ; to be more detailed, the flanks 

 are inflated, and in the region of a rib bicarinate, corresponding to the 

 umbilical and lateral tubercles; venter tricarinate, corresponding to the 

 keel and the two ventro-lateral tubercles; dorsal midline with a slight 

 excavation, into which the next inner coil fits. Number of volutions six 

 or more. At the 6 mm: stage (about four volutions) the shell is prac- 

 tically without ribs. The ribs then come in as straight, broad, low, wedge- 

 shaped elevations, thicker and higher at the marginal end, where the 

 pair of marginal tubercles is an elongate prominence with a slight tuber- 

 culate swelling at each end. Within the next half volution the umbilical 

 tubercles have become prominent, and the marginal tubercles are as tall 

 as the keel, giving to the cross-section of the volution a nearly square 

 aspect, instead of the triangular section which exists in the younger stages, 

 in which the keel is still prominent. The sutures coincidently increase in 

 complexity. 



SUTURE : The siphonal lobe is nearly quadrate. It is bifid and each 

 lobule is relatively simple. The first saddle is broad and is twice bifid. 

 The remainder of the suture is invisible on the flanks of the individuals 

 at hand, which are preserved as pseudomorphs of coarse grained hematite 

 which has replaced all structures and destroyed the details. From the 

 ends of fragments broken along suture lines it is inferred that the first 

 lateral lobe is lower than the first saddle and about two-thirds as broad; 

 terminally it is divided and possibly is trifid. The second saddle is lower 



