LOWER HELDERBERG ROCKS. 3»l 



Fig. 1 a, 6, The anterior and posterior sides of a small specimen. 



Fig. 1 c. Enlargement of the surface, showing the undulating transverse lamellae and the 

 fine longitudinal striae. 



Fig. 2 a, b. Views of the anterior and posterior sides of a larger individual. 



Fig. 3. A larger individual, in which the transverse lamellae are much subdued : the lon- 

 gitudinal ridge is an accidental feature. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Helderberg mountains, Albany county. 



Platyceras spirale (n.s.). 



Plate LXIII. Fig. 4, 5, 6, 7, 8, 9. 



Shell spirally ascending : apex consisting of a single minute close 

 volution, below which are one or two widely separated and gradually 

 enlarging volutions : aperture spreading, rounded or broad oval j peri- 

 stome sinuate. 



Apex and upper part of the shell smooth, or with only fine transverse 

 striae, more or less distinctly plicated on one side below, with strong 

 lamellose undulating striae. 



This species presents considerable variation during its different stages of growth, 

 and in the degree of development of the plications; but there seems to be no real 

 specific distinction among the specimens examined. 



Fig. 4. A young individual of this specie?, in which no plications are developed. 



Fig. 5 a. A larger specimen, in which the plications are very strongly developed on the 

 right posterior side of the shell. 



Fig. 5 6. The posterior side of the same shelL 



Fig. 6. A specimen of equal size with the preceding, in which the plications are but par- 

 tially developed, or shown principally in the undulations of the striae of the 

 surface. 



Fig. 7. A similar specimen which is smooth on the anterior side, and showing only two or 

 three plications on the posterior side. 



Fig. 8. A more elongated specimen, with strongly developed plications. (The upper part has 



been broken off, and presents some imperfection at the junction of the two parts.) 



Fig. 9 a. The anterior side of another specimen, in which plications are but faintly developed. 



Fig. 9 6. The posterior side of the same. The expansion towards the aperture is increased 

 beyond the natural width, by compression. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Albany and Schoharie counties. 



