LOWER HELDERBERG ROCKS. 311 



The species which present themselves in this group of strata, and in 

 the succeeding Upper Helderberg rocks, may be conveniently divided into 

 two or three groups, presenting certain general characters by which they 

 can be distinguished. These distinctions, however, are perhaps not sus- 

 tained in the more intimate relations of the shells. 



In the grouping of the species in their succession in tlie several plates, I have 

 endeavored to bring together in some degree the forms more nearly related; begin- 

 ning with those resembling Platyostoma in form, and progressing to the opposite 

 extreme with as much regard to systematic arrangement as was possible under the 

 circumstances in which the materials were collected, and the long intervals which 

 elapsed between the completion of the earlier and the later parts of this portion 

 of the work, and with constantly increasing collections. 



Platyceras veiitricosuiii. 



Plate LVI. Fig. 1 - 4 & 8 ; and Plate LVII. Fio. 4. 

 Platycerat ventricosum : Conrad, Annual Report on the Palseontology of New- York, 1840, p. 206. 



*• Shell ventricose : aperture very large and campanulate ; volutions 



" three, contiguous, depressed below the upper margin of the whorl." 



The shell is obliquely ovate, spreading rapidly from the apex, and 



becoming extremely ventricose below ; aperture campanulate, the lip 



in contact with the spire, and sometimes strongly reflexed. 



Surface marked by fine transverse or concentric lamellose striae, which 

 are somewhat undulated and rarely finely cancellated by faint revolving 

 striae. 



This species is a comparatively common form in the upper part of the Shaly 



limestone, its usual size being that of the specimens figured on Plate lvi ; rarely 



equalling the size of fig. 4, Plate lvii; while I have one specimen which measures, 



from the anterior margin of the aperture across the volutions to the posterior side, 



more than three inches. 



PLATE LVI. 



Fig. 1 a, b. Views of the upper and lower side of a young specimen which is a cast. 



Fig. 2 a. View of a specimen of medium size, loolcing upon the top of the spire. 



Fig. 2 6. View of the aperture of the same, showing also that the spire is not as high as the 

 outer volution. 



