56 
PROCEEDINGS OF THE ACADEMY OP 
these two types will be found to be connected by many intermediate gradations 
of form, when larger numbers of these Cephalopods are known. 
Locality and position. — Great Bend of the Missouri, lower part of No. 4. 
Helicoceras tenuicostatus. 
The fragment upon which we propose to found this species is slender, nearly 
eylindrical, and increases very gradually in size, from the smaller to the larger 
extremity. It makes a remarkably broad sinistral, ascending spiral curve, so 
as to leave the volutions disconnected, and form a large umbilical space, having 
a diameter about four times as broad as that of the largest whorl. The 
siphuncle is of medium size, and occupies a position above the middle of the 
outer side of the volutions. 
The surface is ornamented by rather irregular moderately distinct annular 
costae, which occasionally bifurcate, and pass nearly straight around the whorls 
On the outside of the volutions the costse are stronger than within, and show, 
a disposition to swell out into obscure nodes. Length of fragment 1-64 inch ; 
diameter at larger extremity -49 inch ; do. of smaller end -44 inch. 
We have for a long time past had this specimen in our possession, but always 
supposed it identical with Ilamites 3Iortoni^ (Hall and Meek, Mem. Am. Acad. 
Arts and Sci. N. S. vol. 5, pi. iv. fig. 3 a. c.) which is probably a Helicoceras. 
After a more careful comparison, however, we find the following differences, 
which we think are of specific importance, — in the first place, the costas of the 
shell now before us are less prominent, and encircle the volutions much less 
obliquely than those of II. iMortoni ; while the siphuncle occupies a higher 
position in the dorsum, and the spiral coil of the shell was sinistral, while that 
of H. Mortoni is dextral. In addition to the foregoing, there are well marked 
differences in the details of the lobes and saddles of the septa, which cannot, 
however, be well explained without the use of figures. 
Locality and position. — Great Bend of the Missouri, lower part, No. 4. 
TURRILITES ? UMBILICATUS. 
We have of this species nearly an entire volution, a little more than half of 
which is septate, but not in a condition to show the form of the lobes. It is a 
sinistral shell, the rounded volutions of which are coiled in an ascending spiral, 
nearly or quite in contact, and increase gradually in size, from the smaller to 
the larger extremity. The umbilicus is a little less than the diameter of the 
largest volution ; and the siphuncle occupies a position in the middle of the 
outer side of the whorls. 
The surface is ornamented by rather distinct, annular bifurcating costae, 
which, on the upper side of the whorls, curve first obliquely backwards and out- 
wards from the umbilicus, then forwards and downwards, as they cross the 
dorsum, and on reaching the under side, curve inwards to the umbilicus. There 
are also two rows of more or less distinct nodes passing around the under outer 
side of the whorls, at which the costse usually bifurcate. 
Greatest transverse diameter across the volutions and umbilicus 3-20 inch ; 
breadth of larger end of whorl 1-19 inch ; do. of smaller end 1-06 inch. 
Not having seen the septa of this species, it is with some hesitation we have 
considered it distinct from T. cochleatus of this paper ; though it differs from 
our specimen of that species in having less neatly rounded volutions, much 
stronger nodes and costoe, and proportionably a little smaller umbilicus ; they 
may, however, be only varieties of the same species. 
locality and position. — Great Bend of the Missouri, lower part of No. 4 of 
general section, 
Ancyloceras (Hamites) uncus. 
We have only seen a fragment of this specie, consisting of the curved portion 
of the non-septate, or body part of the shell. It j^s compressed laterally, and 
makes a rather short curve in the same plane, — leaving between the two extre- 
[March, 
