110 DANFORD : NOTES ON THE SPEETON AMMONITES. 
obscured section. None of these species is at all common, 
but I think the least rare is a form with four constrictions, wliich 
is probably sulcosus. 
The higher deposits of this zone are occupied by the 
Simbirskites section of the Olcostephani. These Ammonites first 
appear in C8, and are therefore overlapped by the Hoplites. The 
group here comprises 9 or 10 species, some well defined, others 
harder to distinguish. They are often spoken of collectively 
under the convenient name of Speetonensis, w^hich is therefore 
said to be common. My own experience, however, is that true 
Spee.tone7isis is rather uncommon, as are all the others except 
inversus, suh-inversus, and perhaps concinnus. The two first 
which are superficially very much alike, but differ in the pattern 
of their sutures, present many varieties, with ribs coarse, fine, 
or arranged in the zig-zag fashion already alluded to, but 
always simply and regularly bifurcate, while in concinnus some 
of the umbilical ribs, especially those of the inner whorls, are 
trifurcate. 
These species occur at almost the lowest horizons occupied 
by this group, but are, according to Mr. Lamplugh's " Table 
of the Speeton Cephalopoda," preceded by Payeri and versi- 
color, and succeeded by progrediens, speetonensis, discofalcatus, 
umhonatus, and Decheni. The two last I have found in the 
topmost beds of this zone, discofalcatus still higher, at the 
base of the next zone. 
In a recent memoir on the Simbirskites group,* Prof. Pavlow 
shows that the specimen with 30 umbilical ribs, described and 
figured in the "Argiles de Speeton" (p. 147, PI. XL, fig. 22) 
as Olc. {Simbirskites) discofalcatus, Lahus. is not that species, 
which has only 19-21 umbilical ribs, but is Simbirskites Phillipsi, 
which has 24-30. 
A specimen that is apparently of the true discofalcatus 
type, mth only 19 ribs, was lately found by Prof. P. F. Kendall 
* " Le Cretace Inferieiir de la Russie et sa faune," Moscou, 1901, 
p. 78., PI. VI., figs. 1, a, b, c, d ; PI. VII., figs. 2, a, b, c ; 3, a, b, c. 
