CEPHALOPODA OF THE CEETACEOUS 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, PI. xxxiv, Fig. la! I have 

 not seen any indications of such a feature on any New Jersey examples of 



