83d CRETACEOUS PALEONTOLOGY. 



Family PLAOBNTIOBRATIDAE. 



Genus Pi^acenTiceras Meek. 

 Placenticeras placenta (DeKay). 



Plate CIV., Fig. 6; Plate CV., Fig. i. 



1828. Ammonites placenta DeKay, Ann. N. Y. Lye. Nat. Hist., 



vol. 2, p. 278, pi. 5, fig. 2. 

 1830. Amnwnites placenta Mort., Am. Jour. Sci., ist ser., vol. 



17, p. 279; vol. 18, pi. 2, figs. 1-3. 

 1830. Ammonites placenta Mort., Jour. Acad. Nat. Sci. Phil.^ 



ist ser., vol. 6, p. 195, pi. 5, fig. 4. 

 1834. Ammonites placenta Mort., Synop. Org. Rem. Cret. Gr. 



U. S., p. 36, pi. 2, figs. 1-2. 

 1861. Ammonites placenta Gabb, Synop. Moll. Cret. Form., p. 



71 (15), (in part). 

 1864. Ammonites placenta Meek, Check List Inv. Foss. N. A., 



Cret. and Jur., p. 25 (in part). 

 1868. Ammonites placenta Con., Cook's Geol. N. J., p. 730. 

 1892. Ammonites {Placenticeras) placenta Whitf., Pal. N. J., 



vol. 2, Monog. U. S. G. S., vol. 18), p. 255, pi. 4, fig. 



I, Pl- 41, %s. 1-2. 

 1903. Placenticeras placenta Hyatt, Monog. U. S. G. S., vol. 



44, p. 211, pi. 39, figs. 3-6, pi. 40, figs. 1-2. 

 1905. Placenticeras placenta Johns., Proc. Acad. Nat. Sci. Phil., 



1905, p. 27. 



Description. — Shell attaining a large size, full-grown indi- 

 • viduals reaching a diameter of 400 mm. or more; subdiscoidal or 

 lenticular in form with a deep and distinct though rather small 

 umbilicus, the sides of which are gently rounded to the lateral 

 surface of the volutions, only a small portion of each of the 

 inner volutions exposed; venter narrow and distinctly flattened 

 in the casts of young individuals and bordered on each side by a 

 row O'f distinct nodes which alternate in position on the two sides, 

 in the larger and more mature individuals the venter becomes 



