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prematurum had no descendants, even a tyro would begin to won- 

 der why it bore the same generic title as the existing species. 



The limits of time and space have not enabled me to follow out 

 each genetic series in this paper, but it will be readily recognized 

 by naturalists that shells having such very different forms in their 

 younger stages must have belonged to different phyla. I have, 

 however, tried in the introduction and in the parts treating of 

 the history of the impressed zone to discuss the facts and arrange 

 them in more intelligible form than is practicable in the following 

 descriptions. 



The families into which genera have been assembled are entirely 

 provisional since more information with regard to the genealogy of 

 the forms is needed before any satisfactory results can be reached 

 in defining these larger groups. 



NAUTILOIDEA. 



Tarphyceratid^e. 



This family includes shells which had elliptical whorls with gyro- 

 ceran mode of growth or subquadragonal whorls with nautilian 

 mode of coiling, the venter narrower than the dorsum in most 

 forms. The shells comparatively smooth, the sutures with ventral 

 saddles or only slight lobes, shallow, broad lateral lobes and either 

 saddles or faint lobes on the venter of free whorls. The siphuncle 

 is ventrad of the centre. 



The genera are as follows: Tarphyceras, Aphetoceras, Delto- 

 ceras, Pycnoceras, found exclusively in the Quebec faunas older 

 than the Chazy ; Eurystomites and Barrandeoceras,* found in both 

 the Quebec faunas and the Lower Silurian ; Planctoceras and Falcili- 

 tuites, found only in the Lower Silurian. 



Tarphyceras^ n. g. 



This genus has heretofore been confounded with Eurystomites by 

 Schroder, the species being found together and resembling each 

 other in general aspect. It differs, however, from that genus in 

 having a more discoidal form, more numerous and more slowly 

 growing whorls, in length of living chamber, in form, aperture, 

 and other characters. 



*In my Genera of Foss. Ceph. I included this genus under the title of Nautilidae. 



t Tap<pbs, close. 



