CEPHALOPODA OF THE CRETACEOUS MARLS. 259 
of three chambers, retaining two lobes and two sinuses each of two of them, 
and a single lobe and sinus of the third; the other fragment, a cast of 
several lobes and sinuses of chambers from near the umbilicus. They 
represent a specimen of large size, probably not less than 7 or 8 inches in 
diameter. 
Fig. 1.—Placenticeras (Sphenodiscus) lenticulare (Owen) Meek. (From U.S. Geol. Surv. Terr., vol. 9, p. 473.) 
In external features this species is much like Am. (Placenticeras) placenta 
De Kay, but is a little more spreading at the umbilicus, although the margin 
of the volutions are closer together, and it is usually sharply rounded on 
the back, instead of narrowly flattened as in that one until it is quite well 
grown, often to a diameter of 7 or more inches before it becomes rounded. 
Owing to the more spreading form of the umbilicus in this one, the point of 
greatest diameter is nearer to the middle of the width of the volution than 
in P. placenta, where it is quite near the inner margin. In the western forms 
of P. lenticularis there are sometimes very indistinct and distant folds on the 
surface, which radiate from the umbilicus, as shown by Mr. F. B. Meek on 
his figure in the Invert. Paleont., as above cited, Pl. xxxiv, Fig. la. I have 
not seen any indications of such a feature on any New Jersey examples of 
