LOWER HELDERBERG ROCKS. 
313 
Fig. 7. A dorsal view of a larger individual which is compressed upon the outer volution, 
showing the narrow linear sinus near the aperture, which, from the bending of 
the striae nearer the apex, must have been wider at that period of growth. 
This shell has, in a great degrfee, the character of a Pleurotomaria, but, 
as far as can be determined, is destitute of a columella. 
Plate lyii. Fig. 2. Yiew of the spire or a specimen of the same species. 
Geological position and locality. In the shaly limestone of the Lower Helderberg 
group : Helderberg mountains, Schoharie, Becraft’s mountain, etc. 
Flatyceras billiiiirsi (n. s ). 
Plate LYII. Fig. 1 a, b. 
Shell subglobose, gradually and uniformly enlarging from the apex to the 
third volution, below which it is abruptly expanded, becoming very 
ventricose; the aperture nearly circular, campanulate. Spire elevated 
above the plane of the outer volution. 
Surface marked by fine lamellose striae, which, on the lower part of the 
shell, are even and scarcely undulating; while the upper part of the 
shell is subcarinate, and the striae are abruptly undulated. 
This species is very peculiar in its contour and in its surface markings. The first 
three volutions present, on the upper side towards the apex, several obtuse undefined 
angles which are resolved into a single one below, over which the striae bend very 
abruptly backwards, indicating a deep and wide sinuosity at this stage of growth. 
At this point the volution suddenly expands; and the surface of the specimen 
being here broken, the gradual obliteration of the sinus is not traceable; but on the 
lower half of the last volution, the striae are simple, and with no perceptible un¬ 
dulation marking the place of the original marginal sinus. There is, likewise, on 
the umbilical side, a sharply defined sinus on the first volutions. 
In a species of this kind, the earlier stages of growth give no evidence of the 
mature form of the shell; and it must be admitted that determinations of species 
in this genus, where founded on one or two specimens without a knowledge of the 
changes which it may undergo in the different stages of growth, are liable to error. 
Fig. 1 a. Yiew of the aperture of this species. [ The striae on the lower side of the figure 
are very incorrectly represented.] 
Fig. 1 b. View looking upon the spire of the same specimen, the upper part of which shows 
the subangular form and the sinuosity of the striae. 
Fig. 1 c. For description, see Strophostylus rotundatus. 
Geological position and locality. In the upper part of the shaly limestone : Be¬ 
craft’s mountain. 
