CRETACEOUS FOSSILS. 67 



I should have hesitated in considering all of these forms as one species, had I 

 not had a large series for study. In every case the septum is the same, and there 

 are many specimens showing intermediate characters between those specially men- 

 tioned above. 



A. Batbsii. 



PL 13, Fig. 16, and 10 a, b. 

 {A. Batesii, Trask. Proc. Cal. Acad. Nat. Sciences, 1855, vol. i, p. 40.) 



Shell discoidal. Whorls eight or nine, not enveloping, merely 

 in contact for about a third of the transverse width ; section nearly 

 circular, slightly sinuous on the ventral side; sometimes the 

 dorso-ventral diameter being greatest; at others, especially in old 

 shells, the transverse diameter is longest. Surface marked by 

 numerous fine, rather sharp, elevated ribs, crossing from the in- 

 terior of the umbilicus obliquely forwards over the dorsum. In 

 some specimens the interspaces are marked by fine revolving 

 lines. In others these lines are absent. 



Septum composed of a dorsal lobe and three on the side, Dor- 

 sal lobe deeply divided, each branch bearing two lateral ones ; 

 above the origin of these are two others. Dorsal saddle divided 

 into two bifurcate branches. Superior lateral lobe a third larger 

 than the dorsal, bifurcate at the end, each branch divided into 

 two smaller, and' these again divided ; above the origin of the 

 large branches are a large and a small spur on each side. Supe- 

 rior lateral saddle like the dorsal. Lateral lobe of the same pat- 

 tern as the superior lateral, but a third smaller! Inferior lateral 

 saddle like the others, but smaller and a little less minutely 

 divided. Inferior lateral lobe simply divided into three or four 

 dentate spurs. 



Figure, natural size of the specimen. One broken specimen, showing none of 

 the body chamber, measures fifteen inches in diameter ; and, when perfect, must 

 have been at least twenty-two inches across. In this specimen the end of the 

 whorl measures five and a half inches in height and seven and a half in transverse 

 diameter. The width of the umbilicus, measured from the sutures, is seven 

 inches. This measurement is not in proportion with the figure given ; the whorls 

 increasing more rapidly in size as the shell becomes larger. 



