BEEKMANTOWN AND CHAZY FORMATIONS OF CHAM PLAIN BASIN 433 



are largely those of degree of development of certain features ; and 

 indeed there exist apparently as many transitional as typical forms. 

 Of the species here referred to the Orthoceratidae, O . lentum 

 and O . progress um can be placed with little hesitation to 

 Orthoceras s. str. ; in regard to O . vagum some doubt is pos- 

 sible on account of the long, uniformly thin shape which suggests a 

 reference to Protobactrites. Since the latter is defined as having a 

 tubular siphuncle and our form has a slightly nummuloidal siphuncle 

 we have preferred to leave it with Orthoceras. Geisonoceras 

 s h u m a r d i has been referred to that genus on account of its 

 somewhat larger rate of growth and relatively smaller size of 

 siphuncle. 



Orthoceras lentum, sp. nov. 



Plate 14, figure 1-3 



This species is based upon two fragments representing different 

 growth stages of the conch. These show certain characters so 

 greatly different from those of the other 

 Chazy orthoceracones that we have little 

 hesitation in seeing a new type in them. 



Description. Slender gently curved 

 conch. Length of type specimen only 26 

 mm but its small rate of growth (but Fig m 0rthoceras len . 

 y 2 mm in 15 mm) indicates the attain- ^L^' ^'^^^^ 

 ment of considerable length in considera- 

 tion of the width of the specimen (18 mm) ; curvature of the frag- 

 ment very small, the hight of the arc not quite amounting to 1 mm. 

 Section circular. Cameras shallow (6 in the space of 20 mm) ; 

 sutures nearly straight transversal ; septa thin and little concave, 

 their depth a little more than two thirds that of the cameras. 



Siphuncle centren, small, 1 mm in the smaller and 2 mm in the 

 larger fragment (one ninth the width of the conch) ; the septal 

 necks very short, ringlike ; the interseptal segments slightly expand- 

 ing, nearly tubular. 



Deposits of carbonate of lime which do not seem to be due 

 to secondary incrustation form a cylinder around the siphuncle 

 and extends thence along the septa thinning out near the outer wall. 

 The living chamber has not been observed. The surface appears to 

 have been smooth. 



Position and locality. The type specimens have been collected 

 by Prof. G. H. Hudson in the dove-colored Chazy limestone ex- 

 posed along the east shore of Yalcour island. 



