72 



NORTH AMERICAN INDEX FOSSILS. 



71. T. planorbiformis Conrad. (Fig. 1283.) Ordovicic. 



Larger than preceding; whorls more compressed vertically, with 

 deeper, impressed suture, and straight, ventral, hyponomic sinus. 

 Surface with longitudinal strise. 



Lorraine of New York and Canada. 



XXIIL DiscocERAS Barrande (Lituites of American authors.) 



Discoidal, costated nautilicones, the costse as in Plectoceras ex- 

 cept the gerontic stage. Young like Trocholites. Differs from 

 Plectoceras in its less rapidly enlarging whorls. Ordovicic-Siluric. 



^2. D. graftonense M. and W. Siluric. 



Whorls scarcely impressed ; costse weak, bending obliquely back- 

 ward, and rather closely crowded. Siphuncle subdorsan. 



Niagaran of Wisconsin. 



73. D. marshii (Hail). (Fig. 1284.) Siluric. 



Fig. 1284. Discoceras marshii, lateral and profile views, X 

 Mus. Rep.) 



(After Hall, 20th 



Venter flattened ; costse sharp and strong, forming a deep, back- 

 ward curve on the venter; interspaces broad and concave. 

 Niagaran of Illinois, Indiana, Kentucky, etc. 



XXIV. Plectoceras Hyatt. 



Gyroceracones and discoidal nautilicones of quadrate section 

 with the abdomen narrower than the dorsum; surface costated, the 



