524 Contributions to Invertebrate Palaeontology. 



GASTEROPODA. 



Genus PLATYCERAS Conrad. 

 Platyceras sqiialodens. 



Plate VII, figs. 6 and 8. 



Platyceras squalodms Whitf., Ann. N. Y. Acad. Sci., March, 1882, p. 202. 



Shell small, sharply conical vrlien 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 concave. Aperture narowly 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 strife 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 and Locality. — In the Upper Helderberg limestone, 

 at Columbus, Ohio. Collection of Columbia College. 



Genus DEXTALirM Linueus. 



Deutaliuin Martini. 



Plate VII, fig. 10. 



DentaUum 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 

 throughout the length ; cylindrico-conical in form, and circular in a transverse 

 section. Surface marked only by encircling strije, which form rather broad 

 undulations on the shell, and are strongly arched forward on the inner side of 

 the curvature, showing that the lip of the shell has been somewhat extended 

 on this side of the aperture. Shell-substance thick. 



The 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 considerable, being deflected fully an eighth of an inch from a 

 straight line within the length of the specimen when tested on the 



