240 PSEUDOCERATITES OF THE CRETACEOUS. 
PLACENTICERAS ORBIGNYANUM (Geinitz). 
Ammonites vibrayeanus Geinitz, 1843, Verst. v. Kieslingwalde, Pl. I, fig. 8. 
Ammonites orbignyanus Geinitz, 1850, Quadersand. in Deutschl., pl. 14. 
Ammonites orbignyanus Drescher, 1863, Zeitschr. Deutsch. geol. Gesell., Vol. XV, 
fig. 8. 
Ammonites orbignyanus Fritsch and Schlénbach, 1872, Ceph. bohm. Kreide, pl. 10, 
fios. 4, 5. 
Ammonites orbignyanus Geinitz, 1875, Elbthalgeb. in Sachsen, pl. 36. 
Placent. fritschi Grossouvre, 1893, Ammonites Craie supérieure, pl. 5, figs. 1, 2. 
This species has fold-like costae and broad venters in the young, as 
figured by Fritsch and Schlénbach and by Geinitz in his Quadersandstein- 
gebirge. One can not say anything definitely from the figure given in the 
first publication under name of vibrayeanus. The figure in Elbthalgebirge 
also appears to be the young of this same species; consequently Drescher’s 
figure of the suture quoted above must also belong to this species and 
serves to correct the conventional figure given in Quadersandsteingebirge. 
There are three principal saddles, as in other species of this genus. - 
Grossouvre’s figures are excellent and give clear ideas of this species. 
The broad, smooth, convex venter of what is probably the gerontic stage, 
and the young, both having costee like the figures in the Quadersandstein- 
gebirge, appear to show that the French and German forms are identical. 
The sutures as figured are different, but this may be due to the poor 
drafting of the figures in the Quadersandsteingebirge and in the Zeitschrift 
by Drescher. 
Age: Lower Senonian. 
PLACENTICERAS POLYOPSIS (Dujardin). 
Ammonites polyopsis Dujardin, 1837, Mém. Soc. géol. France, No. 2, pl. 4. 
This species has a form similar to pseudorbignyanum, but only in old 
age, the ephebic stage being smooth. The flattening of the venter in the 
gerontic stage is a transformation that may occur in any species of this and 
other genera at the same age and can not be considered to indicate specific 
identity. The ornaments are altogether different. The tubercles of the 
outer row are more or less connected with the inner row by indistinct, more 
or less bifurcated, costee, and the inner row does not change place with age. 
The ventral tubercles are larger and more widely separated than in 
