528 
and the angle of growth so convergent that it becomes difficult, 
perhaps impossible, to attribute the existence of this zone to con- 
tact and pressure of a coiled whorl, unless it was acquired by inher- 
itance through some unknown closely coiled forms. 
None of these specimens have the double impressed zone figured 
in Cranoceras ( Cyrtoceras) depressum, by D’ Archiac et De Verneuil,* 
but I have studied some fragments of this species showing the same 
peculiarity. The two latero-dorsal impressions or faces and the 
central gibbous dorsal face give an outline similar to that of the 
young of the Zrocholites canadense, given in Fig. 24, Pl. iv, of this 
paper. ‘The history of the appearance of this modification in this 
large adult whorl, arising as it does from the direct modification 
of the younger rounded dorsum} without being preceded by the for- 
mation of an impressed zone is, however, entirely distinct from 
that which occurs in the paranepionic substage of Trocholites. In 
» several genera of Carboniferous nautiloids (ex. Asymptoceras, 
Apheleceras) similar faces appear on the dorsum, but the central, 
gibbous dorsal face is fitted into the hollow flute or ventral zone of 
the next inner whorl and is obviously a result of close-coiling and 
adaptation of the plastic dorsum of the growing external volutions 
to the ventral modifications of the inner volution. 
In Solenocheilus of the Carboniferous, however, the whorl has a 
rounded venter and yet notwithstanding this a gibbous dorsal face 
and dorso-lateral concave faces or furrows are formed independently. 
In Cranoceras depressum the origin of the gibbous dorsal face and 
latero-dorsal faces or furrows appears also, so far as the facts go, to 
have been entirely independent of any correlation with the ventral 
surface, which is rounded and gibbous. These characteristics do 
not seem to have had a mechanical origin in any of the shells, so 
far examined, which have the dorsal side free or comparatively free 
from contact. 
A very large and remarkable specimen in the Schulze collection, 
Mus. of Comp. Zodlogy, shows a very short living chamber, which 
has an aperture very broad transversely and with a nephritic out- 
line and apparently very broad and well-marked impressed: zone. 
This species is not a variety of Zecatum, but a distinct species pre- 
cisely similar to D’Archiae and De Verneuil’s figures of Phragmoce- 
ras subventricosum, but the siphuncle is ventral. 
* Geol. Trans. London, 20 ser., vi, Pl. xxix. 
7 This is also figured by Roemer, Harzgeb. Palcontogr., iii, Pl. vi, in a young specimen. 
