432 NEW YORK STATE MUSEUM 



longitudinal section fails to show any but rudimentary septal necks, 

 though we must concede that we have been unable to fully satisfy 

 ourselves upon this point. If the funnels are indeed short as in 

 Orthoceras then this form would be one of the primitive Orthocera- 

 tidae, for which Holm has established the genus Baltoceras. The 

 essential difference of this genus from Orthoceras is according to 

 Hyatt the wide siphuncle. Holm states that it has the habit of an 

 Endoceras but the siphuncular walls of an Orthoceras. 



Genus orthoceras (Breyn.) emend. Hyatt 



The term Orthoceras which practically had been applied to all 

 orthoceraconic forms with the exception of those referred to Endo- 

 ceras, and therefore, as a perusal of Barrande's work will show, 

 has included a most astonishing variety of forms ranging through 

 all possible variations in surface sculpture, rate of growth and posi- 

 tion of siphuncle, has been greatly restricted by Hyatt, indeed so 

 much that he stated in 1884 [p. 275] he knew only of two species 

 in North America. 



The cyrtochoanitic forms (those with short, outward curving 

 septal necks) have been brought under the families Loxoceratidae 

 and Rizoceratidae ; the annulated and longitudinally ridged forms 

 have been distributed among the families Cycloceratidae and Kiono- 

 ceratidae, and other orthoceraconic forms with compressed oval 

 section, impressed zone and ventren position of the siphuncle are to 

 be referred to new genera which are not yet defined and are to be 

 placed under the Tarphyceratidae. 



Finally only the simplest patternlike forms of Orthoceras with 

 open apertures, uncontracted living chambers and small tubular, sub- 

 centrally located siphuncle are left. 



These have still been subdivided into Orthoceras (Breyn.), 

 Geisonoceras (Hyatt) and Protobactrites (Hyatt). 



In the earliest definitions of the first two genera [1884, p. 275] 

 Orthoceras was conceived as comprising the smooth longicones and 

 Geisonoceras the banded longicones, the transverse markings of the 

 latter being considered as leading to the Cycloceratidae. In Zittel- 

 Eastman's textbook emphasis is laid on the long tapering form of 

 Orthoceras, the larger size of its siphuncle and the more rapidly 

 spreading sides of Geisonoceras. Protobactrites is proposed for the 

 long pencil-shaped orthoceracones. 



It will be easily understood that an infinite number of transi- 

 tional forms between these genera are possible, whose differences 



