Jacket on Orthoccras.—Rncdciuann. 215 
of isea-weeds, between which run, in approximately east-west- 
erly direction, narrow tide channels filled with black mud and a 
rich mass of gastropods and large straight cephalopod shells, 
The latter lay parallel to the direction of the channel and were 
clearly wedged in or drifted in together with the gastropods, 
which often lay piled up behind the Orthoceras shells. 
There is further to be accounted for the existence of forms 
like Orthoceras truncatus Miinster. This was in the habit of 
regularly breaking off the cone of its shell and then mending 
the mutilated apex with a plug. It can not have been sessile, 
but as the crawling, more or less involute forms are supposed 
to have developed from the sessile orthoceratites it must be 
conceded that these truncate orthoceratites were possibly forms 
which had finally taken the step of detaching themselves but 
instead of rolling up, discarded the unwieldy cones. The slight- 
ly curved Oncoceratites probably stand in close connection to 
these forms and are the result of further adaptation to the 
crawling mode of life. At the same time appears the fact that 
these species preferred to discard entirely the greater part of 
the conch instead of rolling it up, to argue for the supposition 
that the cameras were of little or in fact of no use as hydro- 
static apparatus and indeed but a hindrance to movement. 
Our supposition that the orthoceratites may probably have 
planted themselves in the oceanic deposits, does not appear to 
agree well with Jaekel's conclusion that this was the mode of 
existence of nautiloids with contracted apertures as Gomphoc- 
eras and Phragmoceras, as the latter with their breviconic, 
rapidly expanding cones would seem tO' have adapted them- 
selves to an entirely different habit of life as the orthoceratites. 
But it is here to be considered that the breviconic fonn of the 
shells may well have been the result of further complication of 
their structure as Dr. Clarke suggested to me, namely of the 
contraction of the aperture, which invited a lateral expansion 
of the animal, thus producing low and broad living chambers 
and camerse. Furthermore the Ijroad low shell of these forms 
is obviously well adapted to squatting upon *he sea bottom. 
From the contraction of the aperture and the resulting con- 
striction of the arm-bases, Hyatt concludes that the arms could 
not have been powerful enough in these forms for a crawling 
habit and that thev hence were either sessile or swimming. 
