433 
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. 
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 
Schréder, 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 Nautilide. 
t Tapgds, close. 
PROC. AMER. PHILOS. SOC, XXXII. 143. 3 C. PRINTED JUNE 22, 1894. 
