126 



Report of the State Geologist. 



Initial conch. The tuberous swelling of the shell-tube directly above the 

 protoconch is a very striking feature of these shells. Several of our figures 

 show this feature ; on others it is less distinctly marked and this difference is 

 in a large part, if not wholly, due to the fact that this expansion is greater at 

 the sides of the shell than on the dorso-ventral surfaces, and figures in which 

 it is less pronounced are of the lateral aspect of the shells. This increase in 

 diameter is quite abrupt from the protoconch upward for a distance of two 

 air-chambers, and then more gradually decreases. AVhile the surface of the 

 protoconch is smooth, a fine ornamentation begins with the ananepionic stage. 

 This consists in delicate crowded striations crossing the surface with an 

 inclination to the dorsum. 



In my earlier paper on the Early Stages of Bactrites, written before the 

 nepionic growth-phases of Manticoceras and Tornoceras as heretofore 

 explained, had been made out, this swelling of the primitive conch was 

 spoken of as an " OisrcocERAS-like expansion, for its form, usually unsym- 

 metrical, suggests that genus and may afford a key to its phyletic position." 

 Its apparent lack of symmetry we have just explained. We now regard the 

 existence of this inflation of the nepionic shell the most forcible argument yet 

 advanced for the affiliation of Bactrites with Manticoceras, Probeloceras and 

 hence with the goniatites generally. For the full force of this argument the 

 reader is referred to the detailed account of the expanding and contracting 

 nepionic shell in those genera, as given on preceding pages. Again, we 

 may find in the evident asymmetry of the protoconch in Bactrites, with 

 reference to the conch to which it is attached, and in the slightly asym 

 metrical position of the protoconch upon the conch, as indicated in Man- 

 ticoceras Pattersoni and Probeloceras Luther i, additional evidence of genetic 

 connexion. 



The early stages of this species occur in greater abundance and better 

 preservation than those we have referred to the species Bog. aciculum. It is 

 important here to emphasize distinctly the similarities and differences between 

 these primitive stages and that described by Bkaxco as belonging to Bactrites, 

 for the reason above stated, that upon the latter determination is based the 

 generally accepted phylogeny of the goniatites. The Weissenbach specimen, 

 which is not known to have been duplicated, carried, besides the protoconch, 

 five air-chambers, in a part of which the lateral position of the sipho was 

 shown; for this reason it was with propriety referred to Bactrites. 



The form of the protoconch is shown in the accompanying outline taken 

 from Branoo's figure. It is elongate vertically or egg-shaped, slightly nnsym- 



