386 Systematic Paleontology 



Ammonites placenta Conrad, 1868, Cook's Geol. of New Jersey, p. 730. 



Placenticeras placenta Meek, 1876, Rept. Inv. Cret. and Ter. Fossils, Up. 

 Missouri, p. 465, pi. xxiv, figs. 2a-2b. 



Ammonites (Placenticeras) placenta Whitfield, 1892, Mon. U. S. Geol. Sur- 

 vey, vol. xviii, p. 255, pi. xl; pi. xli, figs. 1, 2. 



Placenticeras placenta Hyatt, 1903, Ibidem, vol. xliv, p. 211, pi. xxxix, figs. 

 3-6; pi. xl, figs. 1, 2. 



Placenticeras placenta Johnson, 1905, Proc. Acad. Nat. Sci., Phila., p. 27. 



Placenticeras placenta Weller, 1907, Geol. Survey of New Jersey, Pal., vol. 

 iv, p. 830, pi. civ, fig. 6; pi. cv, fig. 1. 



I>i scription. — " Orbicular. Sides diminishing rapidly from the center 

 to the circumference, where they form with each other a very acute angle. 

 The whole external and internal surfaces marked by numerous minute 

 arborescent sutures. Septum sinuous, smooth, except where it united 

 with the points of the shell. Here the septum is furnished with robust 

 branched tubercles, and corresponding depressions for the reception of 

 similar tubercles from the adjoining septum. Siphunculus conspicuous, 

 cylindrical, and funnel-shaped as it approaches the septa. Placed on the 

 margin nearest the center of the whole shell. Thickness 1.8, presumed 

 diameter 6.5."— DeKay, 1828. 



" 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 

 thickness, which is only a short distance outside of the umbilicus. Aper- 

 ture elongate sagittate ; on a cast before me where the volution has a 

 width, from the dorsum to the umbilicus, of 4-| in., the greatest thickness 

 from side to side is just 2 in., the diameter of the shell being 8|| in. The 

 surface of the shell I have not seen on New Jersey specimens. 



" Septa closely interlocking, the lobes and their sinuses being of pro- 

 portionally small size, but very complicated, varying greatly in this par- 

 ticular 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 satisfac- 

 torily 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 



