GASTEROPODA. 13 



Platyceras rictum. 



PLATE IV, FIGS. 6, 12-17. 



Platyceras rictum, Hall. Descriptions of New Species of Fossils, etc., page 7. 1861. 

 Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 35. 1862. 

 " " " Illustrations of Devonian Fossils : Gasteropoda, pi. 4. 1876. 



Shell very depressed, arcuate, or sometimes obliquely subcorneal, the width 

 equalling and often exceeding the height. Nucleus minute, the spire en- 

 rolled to about one turn and a half, when it abruptly expands, spreading 

 more upon the right posterior side and becoming greatly extended and 

 expanded in front. The dorsal line of the first volution is continued in 

 an oblique angular ridge extending on the left side, and often reaching to 

 the front of the shell giving a peculiar obliquity to the form. Aperture 

 expanded, obliquely ovate ; peristome entire, or with a slight sinuosity on 

 the left posterior side, and sometimes showing the effect of the undefined 

 folds of the posterior margin usually contiguous to the preceding volution. 



Surface marked by undulating concentric stride, and a few broad undulations 

 longitudinally; width of aperture from one inch to one and three-fourths, 

 and a little greater from the anterior to the posterior margin ; height 

 about one inch and varying from three-fourths of an inch to an inch and 

 a quarter. 



Formations and localities. In the Upper Helderberg limestone of Onondaga 

 county, and in the shales of the Hamilton group in Ontario county, N. Y. 



Platyceras echinatum. 



PLATE V. FIGS. 1-t. 



Platyceras echinatum, Hall. Descriptions of New Species of Fossils, etc., p. 10. 1861. 

 «' " Fifteenth Rep. N. Y. State Cab. Nat. Hist,, p. 38. 1862. 



<< " •< Illustrations of Devonian Fossils : Gasteropoda, pi. 5. 1876. 



Bou small, obliquely subovoid. Apex closely incurved for about one volu- 

 tion or one and a half; the body -whorl, from one-half to one volution, 



