96 University of Texas Bulletin- 



Schistoceras diversecostatum nov. sp 

 PL IV, Fig. 1-36 



Shell discoidal, involute, laterally compressed, with slightly rounded 

 flanks and strongly convex venter. Cross-section helmet-shaped, a 

 little broader than high, having the greatest width near the umbilical 

 shoulder. The involution is a little less than one-third of the height of 

 the whorl. The umbilicus is moderately wide and deep ; the ttmbilical 

 shoulder is rounded; the umbilical wall is steep and broad. No con- 

 strictions are visible on the casts. 



The sculpture changes with the age. A specimen with a diameter 

 of 6 mm. seems to be entirely smooth; a larger one (diameter 9.5 mm.) 

 shows traces of strong spiral ribs, but is not well preserved. Another 

 one (11 mm.) as well as some a little larger (pi. IV, fig. 20-25, 27-31) 

 shows at the umbilical shoulder a row of strong and radially pro- 

 longed, rounded nodules separated by broad and shallow interstices 

 with a rounded bottom; both nodules and interstices, as well as the 

 rest of the surface, are covered by fine but very distinct ribs. These 

 lean forward at the umbilical shoulder and on the lower part of the 

 flank, then curve strongly forward with the convexity towards the 

 front where the flank passes into the venter, and then curve again back- 

 ward, with the convexity backward. These lines are crossed by still 

 finer spiral ribs, especially on the ventral portion, while on the flanks 

 they seem to be missing. They are strongest in the interstices between 

 the sinuous transversal ribs. In a specimen with a diameter of 21 mm. 

 the umbilical nodules are still well developed, but the spiral ribs cover 

 also the flanks and cause slight swellings on the transversal ribs, es- 

 pecially on the venter and the upper part of the flanks. A still larger 

 specimen (diameter, 28.5 mm.) shows the row of umbilical nodules, 

 but they are more numerous, narrower and more elongated as well as 

 fainter. The spiral ribs show strongly on the whole flank, producing 

 distinct swellings on the transversal ribs; on the median portion of 

 the venter they are much stronger and separated by wider interstices. 

 There they are real spiral ribs that pass over interstices as well as 

 transversal ribs without producing considerable swellings on these 

 latter ones. On a still larger specimen (diameter, 36.5 mm.) the 

 spiral lines are no longer visible and the sinuous transversal ribs are 

 more numerous and fainter on the smaller portion of the whorl, while 



