40 FIFTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



PLATYOSTOMA LINEATA. fl-f^^'^./^j- 1.0 



Platyostoma lineatall'] : ConraDj Jour. Acad. Nat, Sciences^ Vol.viii, pa. 275, 



pi. 17, f. 7. 



" Transversely subovate, with wrinkled reticulated strise : aperture 

 " orbicular ; spire depressed, or slightly elevated above the top 

 " of the body- whorl." 



Altliough no geological position is assigned to this species, and the figure 

 does not correspond with the greater part of the specimens examined, I am 

 still inclined to regard this as the common form of the Hamilton group, 

 of which I have seen at least two hundred individuals. 



The form is subovate, approaching to subglobose : the spire is always 

 elevated above the body-whorl, though varying in degree. The shell consists 

 of four or five volutions when entire ; but it rarely happens that more than 

 three are preserved, the apex usually being imperfect. The outer volution is 

 very ventricose and regularly convex, a little depressed (but not canalicu- 

 late) below the suture line : aperture orbicular in perfect specimens ; outer 

 lip thin, with a sharp entire margin ; columellar lip thickened, folded, and 

 reflexed over the umbilicus, which, in adult specimens, is entirely closed. 

 Surface marked by fine, nearly equidistant, thread-like, revolving strigg, 

 which are cancellated by fine concentric strise of about the same strength, 

 but unequally distant. Sometimes the striae are bent abruptly backwards 

 upon the back of the shell. In well-preserved specimens, the surface is 

 beautifully cancellated ; and in the worn and partially exfoliated specimens, 

 gome remains of these surface markings are usually visible. 



This species approaches in surface characters the P. turhinata of the 

 Schoharie grit and Helderberg limestone ; but the spire is never so de- 

 pressed, and the aperture never so straight above, nor so extended on the 

 lower side. It has usually a length of one to two inches. A well-formed in- 

 dividual measures one and a half inches long, with a vertical height of about 

 one inch : another specimen, preserving its proportions free from compres- 

 sion, has a length of two inches, with a vertical height from base of aperture 

 to apex of less than one and a half inches. 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 striaa 

 without revolving striae, while these are preserved on the upper part of the 

 shell. A cast of a specimen in the Corniferous limestone from Batavia has a 

 length of more than three inches, while the vertical diameter of the aperture 

 does net exceed one inch and a half ; and the specimen bears no evidence 

 of compression. The specimens which I have seen from Ohio and the West 

 are casts in limestone, and do not preserve the striae. 



Geological formation and locality. In the Upper Helderberg limestones 

 throughout the limestone range from east to west in New- York, and in the 

 Hamilton group in the western part of the State. 



[ August, 



