ORISKANY SANDSTONE. 475 



Fig. 4. A specimen where the peristome is more expanded. 



Fig. 5 a, 6. A specimen in which the peristome is free and not expanded. 



Fig. 6 a, b. A larger specimen, in which the peristome is expanded and free. 



Fig. 7, 8 & 9. Specimens in which the peristome is not expanded, or hut slightly expanded 

 and essentially free. 



Fig. 10 a, b. Two views of a large specimen with a free peristome and small umbilicus. 



Fig. 10 c. A specimen with expanded aperture, the peristome adjacent to the body volution, 

 but still free. The umbilicus is much larger than in the preceding specimen. 



The three last figures present nearly the maximum of size observed among the specimens 

 from the Oriskany sandstone in Maryland, and it has not been seen in the same position in 

 New York. 



Geological position and locality. In the Oriskany sandstone : Cumberland, Md. 



Platjceras ventricosum. 



Plate CXVIII. Fio. 8-9. 

 Platycercu ventricosum : Conkad. See page 311 of this volume. 



To the description already given, may be added : 



Volutions contiguous throughout, or the last one free ; peristome con- 

 tinuous or interrupted, free or in contact with the body volution, 

 sometimes abruptly expanded at the margin. 



The surface markings are rarely preserved in any considerable degree of perfec- 

 tion in the silicified specimens. 



Fig. 3 a, 6. Views of a young specimen in which the peristome is not continuous ; the body 

 whorl encroaching on the aperture, with an attenuated film of the labrum covering 

 it, the lower side curving into the umbilicus, and the margin below the volution 

 reflexed nearly parallel with the axis. In the figure, the labrum is too distiuctly 

 shown upon the body volution. 



Fig. 4 & 5. Views of a small specimen of the usual form, with a widely expanded aperture; 

 The peristome is only slightly sinuate from the encroachment of the body volutiont 



Fig. 6 a, 6. Views of a specimen above the medium size, where the volutions are in contact, 

 the peristome free and moderately expanded. 



Fig. 7 a. A larger specimen in which the peristome is continuous, but joined to the body 

 volution and abruptly deflected below, giving the appearance of a coluraellar lip. 

 The figure represents very imperfectly the extent of the sinuosity aad the form 

 of the lip below the volution. 



Fig. 7 b. The upper side of the spire of the same specimen, showing a broad shallow sinuosity 

 in the margin. 



