1374.] 229 [Hyatt. 



the dorsum and abdomen of the same breadth and the latter elevated , 

 into an obtuse ridge. This ridge on the fourth whorl becomes a true 

 keel unaccompanied by slight channels, though there are traces of their 

 formation. The quicker increase of the abdomino-dorsal diameter of 

 the whorl after this period speedily elevates the abdomen and ren- 

 ders the transverse diameter of the shell considerably less on the lat- 

 ter part of the fourth volution, destroying even the faint traces of 

 the channels mentioned above. On the fifth volution the sides be- 

 come slightly convergent, and increase very slowly this convergence 

 on the sixth volution. 



The pilse appear on the early part of the third whorl, as thick, 

 widely separated folds, but rapidly grow into true pilae. The genic- 

 ulse may become tuberculated on the fifth volution or they may 

 remain permanently without tubercles ; there is however great varia- 

 tion with regard to their prominence and their number. 



The duration of the different stages of growth is also varied, in 

 some specimens the abdomen remaining flattened through the keel- 

 forming stage until they reach the first quarter of the fifth volution. 

 In one specimen the pilse are reversed in position, bending posteriorly, 

 but subsequently, begin slowly to change to a natural position on the 

 fifth volution. Quite a number of the specimens from Semur have the 

 pilas but very slightly or not at all developed. None of them seem to 

 reach beyond four and a half volutions, and their small size may pos- 

 sibly be due to the same cause as the absence of the pilse. In some 

 specimens the involution may begin to cover up the genicular of the 

 latter part of the fourth whorl, and in others this process may be de- 

 layed for one or more volutions. 



On the early part of the fourth volution the abdominal lobe is con- 

 siderably longer than the superior lateral lobes, which in turn are 

 shorter than the inferior laterals. The inferior lateral cells are 

 slightly deeper than the superior laterals. On the latter part of the 

 same volution the abdominal lobe has increased in length and the 

 superior lateral cells are shallower ; the inferior lateral cells be- 

 come deeper proportionally and the lateral lobes are nearly equal 

 in length, the inferior laterals still remaining, however, slightly the 

 longest. The minor lobes are very minute and remain so, the sutures 

 having a comparatively even outline for that reason. In some speci- 

 mens the lengthening of the abdominal lobe continues until the su- 

 perior lateral cells are almost obliterated ; in others the lobes and 

 cells may remain with about the same proportions as in the stage of 



