NATURAL SCIENCES OF PHILADELPHIA. 
57 
mities, a free space equalling about half the greater diameter of the larger 
limb ; both ends are then extended in the same direction, and, apparently, 
nearly parallel. 
The surface is ornamented by strong, rather angular more or less oblique 
annular costae, which occasionally bifurcate on the sides, and are much more 
prominent towards the dorsal, than the ventral side. There are, also, two 
rows of rather small nodes on each side of the dorsum, placed on the costse ; 
those nearest the dorsum being more prominent than the others, which merely 
consist of a slight swelling of the ribs. 
Diameter across the curve 3-92 inches ; greater diameter, transverse section 
of larger limb 1-73 inch ; smaller diameter of do. 1-10 inch. 
We are unable to determine from this fragment whether it is an Anajloceras 
or a Hamite. 
Locality and position. — South fork of Cheyenne River, near base of Black Hills. 
No. 4, of the general section. 
Ammonites cordiformis. 
Shell, when young, rather compressed, but becoming much more convex with 
age ; dorsum distinctly carinate, in small specimens, much more obtuse in the 
adult. Umbilicus one half to one third as broad as the outer whorl ; propor- 
tionally smaller in large than small individuals ; transverse section of the volu- 
tions distinctly cordate. Surface ornamented by numerous small costaj, which 
are largest near the umbilicus, where they sometimes (in the medium sized 
specimens) swell a little, so as to form obscure transversely elongated, subno- 
dose prominences. A short distance beyond this, they bifurcate regularly, or 
are increased by the implantation of others between, to two or three times their 
number, at the umbilicus. As they approach the dorsum, they curve distinctly 
forward, and in passing over it, impart to the dorsal carina, especially in young 
specimens, a more or less distinctly serrated outline : on the outer volutions of 
large individuals, the costae are almost entirely obsolete, 
The septa, which are not very closely crowded, are divided into five lobes on 
each side, the first four of which are similar in their mode of branching to those 
of A. cordatus of Sowerby ; but they are less deeply sinuous, and present other 
differences in their details. Greater diameter (of an imperfect specimen) 3-30 
inches ; transverse diameter of its outer whorl 1-90 inch : breadth of same 
from umbilicus to dorsum 1*64 inch. Breadth from umbilicus to dorsum of a 
portion of an outer volution of large individual 3-50 inches ; transverse diameter 
of same, near the umbilicus 3-18 inches. 
In many respect this shell is nearly related to the Liassic species A. cordatus^ 
of Sowerby, of which it may be regarded as a representative. All our specimens 
are internal casts. 
Locality and position. South west base of Black Hills, associated with the fore- 
going species. 
Ammonites henryi. 
Shell convex lenticular ; dorsum narrowly rounded, or subangular ; inner 
volutions entirely hidden : umbilicus very small and deep : surface apparently 
without nodes or costae. 
Septa not very profoundly lobed, rather closely crowded, slightly unsymme- 
trical on opposite sides of the shell. Dorsal lobe divided into four principal 
branches, the two terminal of which are smaller than the others, and usually 
tridigitate at their extremities ; while the two lateral divisions are bipartite, 
with more or less sharpely dentate extremities. 
The superior lateral lobe is about the size of the dorsal lobe, and irregularly 
divided into four nearly equal branches, one of which, on the dorsal side, is 
separated from the others by a deeper and broader sinus, than those by which 
they are divided ; while the. other three branches form together a kind of large 
tripartite division : each of the four branches is provided at the extremity with 
1858.] 
