PALEONTOLOGY. 423 



readily distinguished by these characters, as those are distinctly round 

 and ventricose. The only known species approaching this in the angu- 

 larity of the ridge, is M. attenuata, H. & W., of the Chemung group; 

 but this is quite distinct in other respects. 



Formation and Locality. — In the white chalky chert-beds of the 

 Upper Helderberg Group, near Dublin, Ohio. 



GASTEROPODA. 



Genus PLATYCERAS Conrad. 



Plaiyceras squalodens. 



Plate III, figs. 6 and 8. 

 Platyceras squalodens Whitf., Anu. N. Y. Acad. Sci., March, 1882, p. 202. 



Shell small, sharply conical when viewed in a lateral direction, with the apex 

 gently curved anteriorly ; but in a posterior view, the form is narrowly lanceolate, 

 with the dorsal portion rising into a thin, sharp crest or ridge; anterior side 

 rounded and the anterior slope conclave. Aperture narrowly ovate, rounded on the 

 anterior side, widest just above the middle, and extending backward into a narrow 

 point. Surface of the shell marked by fine, hair-like, concentric lines of growth 

 parallel to the margin of the aperture, which is a little bent down anteriorly and 

 posteriorly, and also by a rather faintly marked, but still distinct sulcus, which 

 passes from the apex on the left anterior slope, and over which the striae are 

 slightly undulated, indicating a slight notch in the margin at this point. 



In the narrow and curved lanceolate form of the shell, this species 

 differs very materially from any of the numerous species- of this very 

 monotonous genus, and may be readily distinguished by the sharp dorsal 

 ridge. 



Formation arid Locality. — In the Upper Helderberg limestone, at 

 Columbus, Ohio. Collection of Columbia College. 



Genus DENTALJUM Lmnseus. 



Dentalhun Martini. 



Plate III, fig. 10. 

 Dentalium Martini Whitf., Ann. N. Y. Acad Sci., March, 1882, p. 203. 



Shell somewhat larger than medium size, rather rapidly expanding from the 

 apex to the aperture for a species of this genus, and moderately curving through- 

 out the length ; cylindrico-conical in form, and circular in a transverse section. 

 Surface marked only by encircling striae, which form rather broad undulations on 

 the shell, and are strongly arched forward on the inner side of the curvature, show- 

 ing that the lip of the shell has been somewhat extended on this side of the aper- 

 ture. Shell-substance thick. 



This species attains a rather large size, and expands more rapidly 

 than most species of the genus, reaching a diameter of one-fourth of an 

 inch in a length of less than two inches. The curvature is also consid- 

 erable, being deflected fully an eighth of an inch from a straight line 

 within the length of the specimen when tested on the inner face. There 



