﻿234 INFERIOR OOLITE AMMONITES. 



ever, is the fact that in Gatulloeeras scissum the ventral area is a sunken furrow 

 which the ribs do not cross ; and this feature had appeared long previously in 

 Schlotheimia angulata — a species which can only be traced to a much older branch 

 of the stem which produced Caloceras and the Arietidse. 1 



The manner in which Haug has derived the genus Bumortieria 2 differs in one 

 particular from what I have stated, namely, that he interposes Am. Jamesoni* 

 between Polymorphites polymorphous and Gatulloeeras Vemosse. That Pol. polymor- 

 phous gave birth to Pol. Bronni and Pol. confusus — species with a carina, and with 

 knobs on the outer end of their ribs — which in turn gave birth to Am. Jamesoni, 

 I can readily imagine, but not that Bumortieria or Gatulloeeras is derived from 

 Am. Jamesoni, because, as I have before remarked (pp. 161, 162), I cannot see the 

 reason for the alteration of the complicated sutures of the adult Am. Jamesoni 

 (Hang, op. cit., p. 125) to the simpler sutures of Am. Levesquei. Haug also says 

 that the chief difference between Bum. Vemosse and Am. Jamesoni is that the former 

 is much more strongly evolute. Both these facts are inversions of the usual process, 

 which is a constantly increasing involution accompanied by a gradual progress in 

 complexity of the sutures. 



The similarity which exists between Bumortieria and Am. Jamesoni I very 

 readily admit ; but I account for this similarity by the fact that both are derived 

 from a common ancestor. In effect, Haug's derivation of Bumortieria is more com- 

 plicated than mine ; because he makes out that the ancestral line has passed 

 through two more stages, namely, the Pol.-Bronni-stage, and then, what is 

 practically a reversion, the Am.-Jamesoni-st&ge, before it evolved the true 

 Bumortieria. In my opinion Bumortieria came more directly from Polymorphites 

 polymorphus, or perhaps from Pol. peregrinus ; and the changes necessary to 

 evolve it consisted only in the gradual production of coarser and coarser ribs (Bum. 

 Vemosse), and in time also a keel (Bum. prisca and Levesquei). 



Such, then, are my views concerning the descent of Bumortieria ; with which 

 the genus Gatulloeeras may, for the present, be bracketed. I consider it preferable 

 to treat of all the different species of these two genera in detail, and then in a 

 postscript to add a few notes upon their evolution from one another. 



The extraordinary convergence between certain species of Bumortieria and 

 Grammoceras — a convergence which culminates in the two deceptively similar 

 species Bum. Moorei and Gramm. mactra — renders it very necessary that the 

 descent of Bumortieria and Grammoceras should be clearly traced. Taking only 



1 Hyatt, "Evolution of ArietidcB," ' Proc. Boston Nat. Hist. Soc.,' vol. xvi, fig. 73, p. 166, foot- 

 note ; also " Genetic Relations of the Angulatida?," ibid., 1874, p. 16; also " Genesis of Arietida?," 

 ' Smithsonian Contributions to Knowledge,' vol. xxvi, 1889. 



2 Haug, " Polymorphidse," ' Neues Jahrbuch fur Mineralogie, &c.,' 1887, Bd. ii, p. 120. 



3 Haug includes Am. Jamesoni in the genus Bumortieria, but I omit it. 



