MOLLUSC A— CEPHALOPODA. 27 



1903. Hyatt, Alpheus. Pseudoceratites of the Cretaceous. Mono- 

 graph XLIV. of the U. S. Geological Survey. 



1904. Lasswitz, Rudolf. Die Kreide Ammoniten von Texas. 

 Geologische und Palaeontologische Abhandlungen. N. F., Bd. VI. 

 (X.), Heft. 4, pp. 221-259, 8 plates. 



1905. Hyatt, A., and Smith, J. P. The Triassic Cephalopod 

 Genera of America. U. S. Geol. Survey, Professional Paper No. 40. 



1906. Burckhardt, Dr. Carlos. La Fauna Jurassique de Mazapil. 

 Boletin del Instituto Geologico de Mexico, No. 23 ; 43 plates. 



For descriptions of American species see further : Publications of 

 the Canadian Survey, especially Canadian Organic Remains, and 

 Contributions to Canadian Palaeontology; also Mesozoic Fossils — 

 chiefly by Whiteaves ; Bol. d. Inst. Geol. de Mex., I. (Castillo and 

 Aguilero), and the papers cited under Gastropods (p. 585). 



Artificial Key to the Genera. 



A. Tetrabranchiata : Shelled cephalopods, the shell camerated and external, not 

 enclosed in a calcareous " guard" — animal so far as known with four gills. ..I. . 

 I. Nautiloidea : Shell with simple or gently (rarely strongly, but not angularly) 

 lobed sutures, with concave septa and siphonal funnels of greater or less 



length (rarely with siphonal collars) I. 



I. Protochoanites : Minute orthoceracones with conical septa and flaring living 



chamber I. Volb orth ella . 



I. Holochoanites : Orthoceracones and cyrtoceracones (rarely coiled shells) with 

 (usually) large siphuncle, the funnel extending to or beyond (within) the 



preceding septum *. 



*. Shell straight, cylindrical (orthoceraconic) with siphuncular filling which 



sometimes is alone preserved a. 



a. Siphuncle cylindrical or gradually enlarging, not abruptly expanded 

 in apical portion and not absolutely in contact with shell on one 



side II. 



II. Surface annulated V. Cydendoceras. 



II. Surface of shell not annulated aa. 



aa. Siphonal funnels extending to, but not beyond preceding 



septum f. 



f. With endosipholining a. 



a. "With long non-camerate apical portion (preseptal cone). 

 1 1 A . Proterocameroceras. 

 a. Without long non-camerate preseptal cone. 



II. Cameroceras. 



f. Without endosipholining IV. Endoceras. 



aa. Siphonal funnels extending beyond the previous septum, and 



inserted in preceding funnel III. Vaginoceras. 



a. Siphuncle abruptly expanding in apical portion (preseptal cone) with 

 funnels in contact at one side with shell in camerated portion, result- 

 ing in deflection of suture VI. Nanno. 



*. Shell short, thick and curved (breviconic cyrtoceracone)..VII. Piloceras, 



