136 PROCEEDINGS OF THE ACADEMY OF [1892. 



THE DEVELOPMENT OF THE SHELL IN THE COILED STAGE OF BACUL- 



ITES COMPRESSUS SAY. 



BY AMOS P. BROWN. 



In a former brief communication^ the writer lias noted the 

 discovery of theyoungof the above species in some Cretaceous marl 

 from near Deadwood, S. Dak., and has shoAvn that Bacnlites was 

 coiled in its earlier stages. In the same communication the develop- 

 ment of the suture is illustrated from a generalized Ceratite stage 

 to the adult suture of B. compressus, thus fixing the species of 

 these young forms. Since the above was communicated the writer 

 has been engaged in the study of the development of the shell in 

 the coiled stage and the results of these investigations are presented 

 herewith. 



The coiled stage of the shell consists of two to two and one-half 

 whorls, the diameter of this coiled portion being 0"8to 1" mm. The 

 shell then passes at once into the straight form, either tangent to 

 the spiral or somewhat reflexed in certain cases. By breaking the 

 .shell back from the straight portion to the protoconch, the develop- 

 ment of the shell was made out and the successive stages in this 

 development observed. By an examination of the surface mark- 

 ings of the shell the form and extent of the embryonic shell on leav- 

 ing the egg — the first n?epionic stage — has been determined with 

 considerable certainty. 



The protoconch appears on a front view broadly elliptical in out- 

 line, being 0"55 mm. to 0"60 mm. in axial diameter by 0.45 mm. in 

 vertical diameter; this axial diameter then diminishes in each suc- 

 ceeding whorl and is not again attained until a length of several 

 millimeters of the straight portion of the shell has been developed. 

 Hence the rounded ends of the protoconch may generally be seen 

 projecting beyond the succeeding whorls when the entire spiral 

 portion of the shell is viewed edgewise. The suture line of the first 

 septum is marked by the prominent narrow saddle over the 

 siphuncle which determines that this form belongs to the Angusti- 

 sellati of Branco. The remainder of the first septum is rather 

 simply curved, the lateral saddles of the succeeding septa being per- 

 haps represented by the slight lateral undulations that exist. Seen 

 from the side the protoconch has the form shown in PI. IX, fig. 4, while 



jProc. Acad. Nat. Sci., Phila., 1891, pp. 159-160. 



