GRAMMOCERAS MACTRA. 177 



XXXI, figs. 3, 4), fairly typical in side view, has practically no carina — the edges 

 of the ventral area meet at an angle, which becomes blunt and almost rounded 

 oflTin the last whorl. Another form, y (PJate XXX, figs. 3, 4), shows a much more 

 compressed section than Dumortier's figure. It also has a well-marked inner 

 margin, seen neither in that figure nor in the other specimens. Yet another 

 form, S (Plate XXX, figs. 5, 6), has coarse ribs until what is, for this species, late 

 in life ; and these ribs are not fascicled, but are separated by unequal spaces, so 

 that now and then two or three come closely together on the inner margin. 



The fact is that the various specimens show a gradual progression of develop- 

 ment in different directions. The form S shows the greatest resemblance in 

 ribbing to the supposed parent-form, while the form -y is more removed, for it 

 assumes at 0*7 inches the fine strige which the other does not get till 1-7 inches. 

 It also has a more developed inner margin. The form |3 is actually a senile form, 

 exhibiting — in the fact of its almost uncarinate ventral area — the decay of a character 

 really peculiar to the species. It is this very decay of the carina which helps the 

 convergence to Dmnortieria Moorei. The form a possesses a smaller umbilicus, 

 and shows the tendency of the species to advance in this direction, though still 

 retaining a sharp carina. 



It is most probable that Gramm. madra is the direct descendant from Oramm. 

 striatulum, and that the form S shows the period when greater compression had 

 been obtained, but the coarse ribs had not yet entirely disappeared from the 

 inner whorls. 



The other forms show the development of the latter character, accompanied by 

 the acquirement of different characters in each form. Though Gramm. mactra be 

 regarded as a still further development of the characters of the senile Gramm,. 

 striatulum, it may yet be definitely separated from that species by its very 

 numerous, often fascicled, fine striee, by its compressed shape, and its broader 

 whorls. 



As the literature on the subject will show, Gramm. mactra is far more likely to 

 be confounded with species of other genera, particularly with those of Biimor- 

 tieria, than with forms of its own genus. 



The extraordinary convergence in shape between this species and Bwmortieria 

 Moorei is one of the most interesting features in Ammonite development. To the 

 researches of Dr. Haug^ we are greatly indebted for a better knowledge of the 

 curious convergence between Dumortieria and Grammoceras, and for the correct 

 specific denominations of the various forms. 



The list of synonyms given to the various species of Grammoceras and 

 Dumortieria will abundantly prove the perplexing trouble which they have 



^ "Beitrage Monog. Harpoceras," 'Neues Jahrbuch filr Miner., &c.,' iii, Beil.-Bd., 1S85, and 

 " Polymorphid»," ibid., Bd. ii, 1887. 



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