80 NEW YORK STATE MUSEUM 



Orthoceras crebescens Hall 



Plate 10, fig. 24, 27, 28 ; plate 11, fig. 1-5 



Orthoceras crebescens Hall. N. Y. State Cab. Nat. Hist. 20th An. Rep't. 



1867. p. 354, pi. 19, fig. 1-3 

 Orthoceras crebescens Whiteaves, Paleozoic Fossils. 1884. v. 3, pt 1, 



P- 37 



Orthoceras crebescens Whiteaves, Paleozoic Fossils. 1895. v. 3, pt 2, 

 p. 98 



Not Orthoceras crebescens Hall & Whitfield. Geol. Sur. Ohio. Paleon- 

 tology. 1875. v. 2, pt 2, p. 148, pi. 9, fig. 2 



Among the orthocerata of the lower Shelby dolomite is a small group 

 of specimens which contrast with the others by their large dimensions, and 

 specially the greater depth of the chambers. These must be the relics of 

 the giants in the cephalopod fauna that populated the lower Guelph coral 

 reef. They are all casts of the interior. 



Diagnosis. Conch straight, thin, of large size, the living chamber of 

 one specimen attaining a diameter of about 70 mm, gradually expanding 

 (angle 9 ), cross section circular or subcircular ; surface smooth, faintly 

 striated longitudinally ; sutures transverse with a distinct (ventral ?) lobe, 

 camerae very deep, the sutures lying 9 mm apart in one specimen where the 

 conch has a diameter of 40 mm, and in another where the diameter is 50 

 mm ; septa evenly concave and very deep, the depth being about equal to 

 the depth of the camerae ; siphuncle centren or nearly so, large, nummuloidal 

 in the mature stage, where it passes the septum, about one sixth the diame- 

 ter of the conch ; living chamber possessing the same rate of expansion as the 

 septate portion, repeatedly slightly constricted ; aperture apparently nearly 

 straight. One cast retains faint longitudinal ridges, with flat interspaces. 



Horizon. Lower Shelby dolomite. No specimens have been obtained 

 in either the upper bed at Shelby or Rochester. 



Our materia' agrees in all essential features with the type specimen 

 described by Hall from the Racine limestone, Wisconsin. In vol. 2, 

 Geology of Wisconsin, the species is listed as occurring in the Waukesha, 

 Racine and Guelph beds ; in the Racine beds at the greatest number of 



