130 



Report of the State Geologist. 



"form certainly has the characters of an Orthoceras, but the protoconch is 

 large and like that of the Animonoidea. The shell may be transitional from 

 Orthoceras to Bactrites, but it is probably not a typical form of Orthoceras." 

 At first thought this suggestion seemed to me untenable ; it was hardly 

 probable that a long period of careful exploitation of this fauna would have 

 produced only this single protoconch as the representative of a generic form 

 transitional between Orthoceras and Bactrites; and yet it is conceivable that 

 such differences as are here indicated by the young may be totally extinguished 

 in the later growth stages so that the mature form of the species in question 

 may be before us, though present knowledge does not enable us to recognize 

 it. Professor Hyatt's wise expression thus throws additional emphasis upon 

 the orthoceran affiliations of Bactrites. The present view of Professor Hyatt 

 with reference to the status of Bactrites is clearly expressed in the following 

 words from the work last cited (p. 362), " Bactrites is a perfectly straight 

 form, similar to the members of the Goniatitinae in all important character- 

 istics, especially the siphuncle and septa, and it also has, like the young shell 

 described by Clarke, and all the coiled Animonoidea, a comparatively large 

 protoconch, as demonstrated by Branco * * This same genus includes 

 straight cones like Bactrites {Orthoceras) pleurotomus Barr. (Syst. Sil. pi. 290), 

 which are undeniably transitions to true Orthoceras in their striae of growth 

 and position of siphuncle. There is, therefore, convincing evidence in the 

 structures of these Silurian shells that the Animonoidea, with their distinct 

 embryos, arose from the orthoceran stock and passed through a series of forms 

 in times, perhaps, preceding the Silurian, which were parallel to those charac- 

 teristic of a number of genetic series among NautiloiJea, viz., straight, arcuate, 

 gyroceran and nautilian." 



Although this inference as to the origin of Bactrites from orthoceran 

 ancestry was to a considerable degree based upon the characters of the primi- 

 tive shell in that genus as described by Buanco, yet such additional facts as we 

 have been able to bring out do not fail to confirm it. We find strong indication 

 of orthoceran affiliation first, in the globular protoconch and its similarity to 

 the Orthoceras-Yike shell of the Styliola limestone ; secondly, in the character 

 of the sipho which is truly intra-marginal in Bactrites. On the other hand, no 

 true goniatite has the sharp constriction about the entire periphery at the 

 close of the protoconch, although it is approached by Mimoceras. The early 

 erect shell in Mimoceras, and the swelling of the nepionic shell in Bactrites 

 which is reproduced in several genera of goniatites, Torxoceras, Mantu ockras 

 must be regarded as fortifying the goniatitine affinities of the genus. 



