CEPHALOPODA OF THE CRETACEOUS MAELS. 255 



Subgenus PLACENTICERAS Meek. 

 Ammonites (Placenticbeas) placenta. 



Plate XL, Fig. 1, and Plate xli, Figs. 1 and 2. 



Ammonites placenta Dekay : Ann. N. Y. Lyceum Nat. Hist., vol. 2, p. 378, PI. V, 



Fig. 3; Morton, Jour. Acad. Nat. Sci., Phila., 1st ser., vol. 6, p. 195 ; Am. 



Jour. Arts and Sci., vol. 18, PI. ii. Figs. 1-3; Synopsis, p. 36, PI. ii, Figs. 



1 and 3 ; G-abb, Synopsis, p. 15 ; Meek, Check List Cret., p. 35 ; Geol. Surv. 



N. J., 1868, p. 730. 

 Placenticeras placenta (Morton) Meek: Invert. Pal. U. S. 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. Dorijum 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 4J inches, the greatest thickness from side 

 to side is just 2 inches, the diameter of the shell being 8f 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 inner 

 ones are only serrate on the sides with a perceptibly swollen extremity. 

 There are intermediate lobes between all the principal ones, except the last 



