CEPHALOPODA OF THE CRETACEOUS MARLS. 259 
Subgenus PLACENTICERAS Meek. 
AMMONITES (PLACENTICERAS) PLACENTA. 
Plate xu, Fig. 1, and Plate xii, Figs. 1 and 2. 
Ammonites placenta Dekay: Ann. N. Y. Lyceum Nat. Hist., vol. 2, p. 278, Pl. v, 
Fig. 2; Morton, Jour. Acad. Nat. Sci., Phila., Ist ser., vol. 6, p. 195; Am, 
Jour. Arts and Sci., vol. 18, Pl. m, Figs. 1-3; Synopsis, p. 36, Pl. u, Figs. 
1 and 2; Gabb, Synopsis, p. 15; Meek, Check List Cret., p. 25; Geol. Surv. 
N. J., 1868, p. 730. 
Placenticeras placenta (Morton) Meek: Invert. Pal. U. 8. Geol. Surv. Terr., p. 465. 
Shell attaining a large size, subdiscoid or lenticular with a deep and 
distinct umbilicus, the sides of which are gently rounded to the surface of 
the volution, exposing only a very small portion of each of the inner volu- 
tions within it. Dorsum of the shell narrowly rounded and the sides of the 
volution gradually diverging from its edge to the point of greatest thick- 
ness, which is only a short distance outside of the umbilicus. Aperture 
elongate saggitate; on a cast before me where the volution has a width, from 
the dorsum to the umbilicus, of 4§ inches, the greatest thickness from side 
to side is just 2 inches, the diameter of the shell being 8? inches. The sur- 
face of the shell I have not seen on New Jersey specimens. 
Septa closely interlocking, the lobes and their sinuses being of propor- 
tionally small size, but very complicated, varying greatly in this particular 
with the age of the shell. The interlocking of the septa is so great in the 
very fine specimen mentioned above that it is impossible satisfactorily to 
trace any single one entirely across the volution. The lobes in the larger 
portion of the volution appear to be ten in number exclusive of the dorsal 
lobe, and to be somewhat smaller than the corresponding sinuses, except 
the second and third. The dorsal or siphonal lobe is very wide and deeply 
forked. The third lateral lobe, or fourth counting the dorsal, is larger than 
any other, with two large lateral processes and a bifid extremity. The 
others are generally trifid to the fifth or sixth, beyond this a few of them 
are -bilateral with two divisions on each side; some of the inner ones are 
long and clavate, with three or four slight projections, while the two imner 
ones are only serrate on the sides with a perceptibly swollen extremity. 
There are intermediate lobes between all the principal ones, except the last 
