3. DEVELOPMENT OF THE SHELL IN THE 

 GENUS TORNOCERAS Hyatt* 



(Plate XXXIV) 



The leading embryonal characters of the genus Tornoceras 

 have been drawn mainly from results obtained in the study of 

 Tornoceras retrorsum von Buch, and allied species from the 

 Devonian of Germany. -I" Probably the best study of any one 

 of the species is that given by W. Branco of T. retrorsum, var. 

 tijpum Sandberger. % The adult features have been deter- 

 mined from the type T. (Goiiatifes) uniangulare Conrad, and 

 other closely related forms. Hitherto, knowledge of this 

 species has not been sufficient to give a reasonably full 

 diagnosis of the genus in its developmental relations, and 

 the results of the following study aim to supply the deficiency. 

 The importance of this is evident, as the characters of the 

 type are of prime consequence, and because T. retrorsum 

 offers some differences in its development, and apparently 

 belongs to one of the more advanced phases in the evolution 

 of the generic stock. Instead of presenting a gradual growth 

 from its simple nautiliform protoconch through several slightly 

 diverging stages, it exhibits, to a degree, the principle of 

 accelerated development, as will be shown hereafter ; while 

 T. uniangulare has a more uniform and complete growth, and 

 is probably one of the initial and most primitive species of 

 the genus. 



* Amcr. Jour. Sci. (3), XL, 71-75, pi. i, 1890. 



t Proc. Boston Soc. Nat. Hist., XXII, Hyatt. Genera of Fossil Cephalopoda, 

 320, 1883. 



X Patreontographica, XXVII. Beitrage znr Entwickelungsgeschichte der 

 fossilen Cephalopoden, 1880. 



