PLACENTICERATID 2. Pale 
line of tubercles. The ventral lines of tubercles persist on the outer 
volution, but are very faint and finally disappear. The same is true of the 
other lines of tubercles, all of which finally disappear in the paragerontic 
stage. The shells of this species do not apparently have the gerontic 
enlargement of tubercles into heavy blunt nodes which is common in 
guadalupe and its allies, sancarlosense, planum, and also syrtale. The sutures 
are like those of syrtale and less complicated than in intercalare at the same 
age, but this appearance is probably largely due to the fact that they are 
less crowded and do not overlap. 
Locality: Black Hills region. 
Age: Fort Pierre group, Upper Cretaceous. 
PLACENTICERAS PLACENTA (Dekay). 
Pl. XX XIX, figs. 3-6; Pl. XL, figs. 1, 2. 
Ammonites placenta Dekay, 1828, Ann. New York Lye. Nat. Hist., Vol. II, pl. 5, fig. 2, 
not fig. 3. 
ras placenta Meek (pars), 1876, Mon. U.S. Geol. Surv. Terr., Vol. IX, 
p. 465. 
Placenticeras placenta Whittield (pars), 1892, Mon. U. S. Geol. Survey, Vol. XVIII, 
pls. 40, 41. 
This species is represented in the Museum of Comparative Zoology 
by a large fragment from New Jersey 101 mm. in diameter from line of 
involution to venter at base of living chamber, and this last is about 
one-half of a volution in length. It is a cast and smooth on the sides, with 
the exception of a line of fold-like tubercles on the depressed umbilical 
shoulders. ‘The ventral zone, even at this stage, obviously fully gerontie, 
is flat and 7.5 mm. broad as estimated. 
The last sutures have not so highly complicated outlines as whitfieldi, 
but the lobes are long and narrow, the first and second laterals highly 
inclined apically. The fourth lateral is about two-thirds as long or only 
slightly shorter than the third lateral lobe. The saddles are not so deeply 
divided as in whitfieldi and broader and more solid. The last two sutures 
are approximated, but the third, although partly preserved, is at the usual 
distance and shows that this is not the same as the western form described - 
by Meek as placenta. The sutures of this large specimen of P. placenta 
have exactly the outlines given by Whitfield in his figure, and a similar, 
