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 degree, the character of a Pleurotomabia, but, 

 as far as can be determined, is destitute of a columella. 



Plate LVII. Fig. 2. View 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. 



Platyceras billingsi ( n. s.). 



Plate LVIL Fto. 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. 



SuKFACE 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 strije 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 plac« of the original mai^inal 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 det«rminations 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. View of the aperture of this species. [ The striae on the lower side of the figure 

 are very incorrectly represented.] 



Fig. 1 6. 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 Stropkostylus rotundahit. 



Geological position and locality. In the upi)er part of the shaly limestone : Be- 

 craft's mountain. 



