22 
PALEONTOLOGY OF NEW YORK. 
worn and partially exfoliated specimens, some remains of these surface 
markings are usually visible, the revolving striae being first obliterated. This 
species approaches in surface characters the P. turbinata of the Schoharie grit and 
Helderberg limestone; but the volutions are never so depressed on the upper 
side, and the aperture never so straight above, nor so extended on the lower 
side. It has usually a length of from one to two inches; young individuals of 
half an inch in length are not uncommon. A well formed individual measures 
one inch and a half long, with a vertical height of about one inch; another 
specimen, preserving its proportions free from compression, has a length of 
two inches, with a vertical height from base of aperture to apex of less 
than one inch and a half. A single extravagant specimen has a length of 
three inches, with a width of aperture of nearly two inches; the body- 
volution, for a distance of two inches from the aperture, is more than 
usually straight, and marked by crowded and unequal concentric striae, 
without revolving striae, while the latter are preserved on the upper part of 
the shell. A cast of a specimen in the Corniferous limestone of Western New 
York, bearing no evidence of compression, has a length of more than three 
inches, while the vertical diameter of the aperture does not exceed one inch 
and a half. The specimens which I have seen from Ohio and the West, are 
casts in limestone, which, not preserving their striae, are identified only by their 
form -and proportions. 
The original description of Plotyostoma lineata is as follows: 
“ Transversely subovate, with wrinkled reticulated striae; aperture orbicular; 
spire depressed, or slightly elevated above the top of the body-whorl.” 
Although no geological position is assigned to the species by Mr. Cone ad, 
and the figure given by him does not correspond with the prevailing form of 
the specimens examined, I am still inclined to regard it as the common form 
of the Hamilton group, of which I have seen several hundred individuals. 
Formations and localities. In the Upper Helderberg limestone throughout the 
limestone range from east to west in New York, and in the Hamilton group in 
the western part of the State. 
