156 PSEUDOCERATITES OF THE CRETACEOUS. 
gibbous sides and have also comparatively flattened or rounded and much 
wider venters. ; 
Having been loaned through the kindness of Dr. Pilsbry another speci- 
men of this species from the collection of the Academy of Sciences, Phila- 
delphia (Pl. XVII, fig. 20), I can state the following additional particulars: 
The diameter is about 82 mm., partly estimated. The living chamber is 
one-half of a volution in length on the periphery, but is much shorter on 
the line of involution, owing to the great apical trend of the aperture. This 
has a broad but very slight sinus on either side and apparently no lateral 
erests on the sides that could be separated from the rostrum. This last, 
however, was broken and could not be decisively determined. The 
specimen is in its gerontic stage and upon the inner parts of each side has 
four heavy folds which disappear near the venter. The bifidity of the 
internal saddles is variable, since in another specimen in the collection ot 
the Museum of Comparative Zoology the sixth saddles were bifid. 
Locality: Avivechi, Sonora, Mexico. 
Age: Fredericksburg group, Comanche series. | 
PROTENGONOCERAS PLANUM n. sp. Hyatt. 
Pl. XVIII, figs. 6-9. 
This species is described from a fragment which would be ordinarily 
insufficient for diagnostic work.. The greatest length of this piece is only 
22.5 mm., the ventro-dorsal diameter of the volution 14 mm., the greatest 
diameter at umbilical shoulders 5.6 mm. Nevertheless, the cast is perfectly 
smooth, the sides flat, and the venter, which is only 1 mm. in width, is also 
smooth, concave, and bicarinate. These characteristics, especially the 
attenuated venter and smooth sides, distinguish it quite sufficiently from the 
preceding species to justify specific separation. There are ten saddles, ail 
narrow, the first symmetrically bifurcated, as in other forms of this genus 
and Hngonoceras. ‘They are entire and from second to eighth more or less 
club shaped, but the ninth is broader and bifid. The tenth and eleventh 
are entire, occupying the umbilical zone. The dorsal zone of impression 
has six saddles, counting the outermost one above mentioned (Pl. XVIII, 
fig. 9). There are ten narrow zygous external lobes (Pl. XVIII, fig. 8) on 
the right side. The first is club shaped and entire, the second to the fifth 
similar, but faintly trifid) The sixth to the tenth are shorter and decrease 
