548 
regarded as an actual characteristic belonging to the shell. I now, 
however, consider it probable that this fragment, in spite of its 
great similarity to the first volution of P/lewronautilus superbus, 
should not be placed with this species, nor regarded as a first 
whorl, but as another new species. I incline much more at present 
to the view that the impression (the author’s dorsal furrow) upon 
the concave side might have originated from a whorl now broken 
away.’ 
Nautilide. 
Without attempting at present to limit the chronologic distribu- 
tion of this family, it is necessary in this connection to make some 
remarks with reference to my observations on the general affinities 
of the genera described in this paper, which are all Mesozoic. 
Digonioceras is obviously the most primitive type yet found in 
the Mesozoic, and the most primitive or most generalized species is 
Digonioceras excavatum, as figured by D’Orbigny. ‘The broad first 
whorl of this species is persistent in adults and so also is the slight 
amount of the involution and the discoidal character of the coil. 
The digonal and approximately nephritic outline of the young in 
the paranepionic is succeeded by a subtrigonal outline in the adult. 
This is substantially paralleled by the development of the species 
of Cenoceras, Cymatoceras, Eutrephoceras and Nautilus. 
All of these are apt in their nepionic substages to bring out the 
nephritic, and in the paranepionic the subtrigonal form of whorl 
with a broad dorsum, converging lateral zones and more or less 
subacute or elevated venter. This occurs even when the nephritic 
outline or some other is assumed in the later stages. 
There is, therefore, in all of these genera some direct reference 
to the form of the ephebic stage of Digonioceras. 
This fact is of great importance in connection with the assump- 
tion made in this memoir, that after the Trias the survivors of the 
Nautiloids are all nautilian shells and bear the marks of their descent 
from close-coiled ancestors and are not directly connectible with 
straight or arcuate types as the nautilian shells of the Paleozoic 
often are. . 
Digonioceras, n. g. 
Digonioceras excavatum, was described in my Genera of Fossil 
Cephalopods, as a member of the genus Endolobus surviving in the 
