82 POMPECKJ. JURASSIC FAUNA OF CAPE FLORA, [norw. pol. exp. 



The whorls gradually grow broader. At a diameter of about 30 — 35 mm. 

 the previously low wall of the umbilicus becomes higher. The greatest thick- 

 ness of the whorls is then situated near the umbilicus, and the outside be- 

 comes broader. The further development of the form is not to be observed 

 in any of the pieces from Cape Flora before me. In the specimen figured 

 on PI. II. tig. 7, the fragments of a body chamber pressed quite flat are to 

 be seen. 



The innermost whorls, and those up to a diameter of about 6 mm. are 

 smooth and without any sculpturing whatever. Then slight wrinkles with a 

 decided flexure forwards, become visible on the flanks. At a diameter of 

 about 10 mm., the wrinkles on the flanks become more distinct, and at about 

 half the height of the whorls generally bifurcate into two lower, less distinct 

 wrinkles, which, bending slightly forwards, run over the outside. At a size 

 of 15 — 25 mm. in diameter, the whorl is covered with 'narrow, rather sharp 

 ribs, distinctly falcate — PI. II. fig. 7 b ^ — ^; near the outside, the ribs show 

 an especially distinct forward flexure. On the outside, the ribs from both 

 flanks run towards one another at an angle of about 90°; they do not, how- 

 ever, meet at an angle, but in a short curve — PI. II. fig. 7 c. On the lower 

 half of the whorls, the ribs are somewhat sharper and higher than in the 

 upper half. 



Some of the ribs run over the whole whorl without any bifurcation; 

 to some extent, however, secondary ribs from the outside are interposed 

 before or behind a primary rib. At half the height of the whorls, or a little 

 nearer the umbilicus, the secondary ribs meet with the primary ribs, or 

 only approach the latter, and terminate indefinitely. In this way more or 

 less distinctly bifurcated ribs are produced. There is no fixed rule in the 

 distribution of the divided and undivided ribs. If a indicates an undivided 

 rib, 6i a divided rib where the secondary rib joins the primary rib from 

 behind, 62 a divided rib where the secondary rib joins the primary rib in 

 front, we have the following formula for the sculpturing on the first half of 

 the last whorl (from 14—20 mm. in diameter) of the specimen figured on 



In this point our form agrees better with the figures given by d'Orbigny than with the 

 imperfect illustration by Nikitin, mentioned above. 



