66 PSEUDOCERATITES OF THE CRETACEOUS. 
SpHENopIScUS LoBATus (‘Tuomey). 
JBL Waly Giese Ja W0G ites il Bg Jel JO jiker, lets, 
Ammonites lobata Tuomey, 1856, Proc. Acad. Nat. Sci. Philadelphia, Vol. VII, p. 168. 
The description given by Tuomey of his Amm. lobata may of course 
apply to another species, but this is the only one I have seen to which 
the following words could have applied, and it comes from the same State 
although from another county: “Shell discoidal, smooth, thin toward the 
circumference;” ‘dorsal (ventral) lobe finely serrate.” These words and 
his reference to the large bilobed saddles as characteristic seems to make 
this name applicable to this species, which is so widely different from 
Sphenodiscus lenticularis and its nearest aftines that no discussion is necessary. 
A fine specimen (PL. VI, figs. 1, 2) in Coll. Nat. Museum, labeled S. 
lenticularis, No. 20577, from Ripley group, Lander’s mill, Tippah County, 
Miss., is 111 mm. in diameter. The last volution at what appears to be the 
aperture measures 59 mm. and the volution immediately opposite in same 
diameter from line of involution to venter is 46 mm. The greatest trans- 
verse diameter is about the middle of the lateral zone and is 21.5 mm. and 
for the smaller part of the volution 15.5 mm. he cast is naked except a 
fragment that shows that it did not have a very thick shell. The inner 
volutions are not entirely covered and the umbilicus is larger proportionally 
than in the large specimen, supposed to be the adult of the same species, 
from Pontotoe County, Miss. There are obscure fold-like coste indicated 
outside of the greatest transverse diameter, which is nearly central; inter- 
nally the surface is slightly concave. There are no umbilical shoulders 
and no flat umbilical zone and the umbilical openings are shallow. ‘The 
shell must have been very thick between the volutions and may have much 
contracted the opening of the umbilicus. There were twelve lobes and 
thirteen saddles on the oldest part of the volution. The flat siphonal 
saddle has a minute saddle in the center and a couple of inflections or 
marginal lobes on either side of this and then at the ends two small 
round saddles. The ventral lobe is very broad and the two arms also 
broad and obscurely trilobate, each lobe being subdivided by a minute 
saddle. The first, second, and third lobes are broad at top and have an 
unequal number of small short branches, as if they were derived from the 
trifid type. They are all probably, however, derived from a bifid type, 
unless exception may be made for the branches of the ventral lobe. 
