602 
The mechanical moulding of the dorsum upon the venter of the 
next inner whorl is shown of course in all of these examples, but it 
can be still better illustrated by such forms as Ophidioceras, just de- 
scribed, and Apheleceras mutabile (Pl. x, Figs. 29-31) and Diorugo- 
ceras planidorsatum (Pl. xii, Figs. 1, 2). These and many other 
examples besides those figured serve to demonstrate that in every 
shell, so far as known, the configuration of the dorsum is absolutely 
dependent upon the shape of the venter, the former being invariably 
a reverse or mould of the latter. The same is also true in the 
earlier stages of the contact furrow in those species that strike and en- 
velop the apex of the conch. 
The number of series which have close-coiled shells, but in which 
the impressed zone is purely a contact furrow, isin the Carboniferous 
even larger than in the Devonian, but it will suffice to refer to two 
extreme examples. Ephippioceras, which is a highly specialized 
species with peculiar sutures and septa and very involute, appears to 
belong in this category, and also Phacoceras. ‘These forms are in 
part figured on PI. ix. 
Similar transitional shells with good-sized or large umbilical per- 
forations are also present in the Trias, and are illustrated in 
Syringoceras granulosostriatum and linearis, Pl. xi. There are 
several other species in the Trias that belong in the same catc¢gory, 
but it is not always easy to get preparations that will establish the 
fact that the dorsal furrow is absent. 
The disappearance of the straight and arcuate types in this period 
together with the transitional nautilian shells has been remarked 
above, and in the course of the following pages this fact will be 
noticeable. In dealing with those types in the Carboniferous that 
possess a dorsal furrow, one is struck by their small number and 
their decisive testimony in favor of the assumption that the dorsal 
furrow is inherited. 
The phylogerontic character of Coloceras globatum is evident 
from the figure of the ananeanic stage on Pl. x, and the comparison 
that may be made with the senile whorls of Vesténautilus koninckt. 
It then becomes obvious that Coloceras belongs to the same genetic 
series as Vestinautilus, but that it inherits degenerative characters 
at an early stage. It is in other words a degenerate form with a 
highly accelerated development of the gerontic or degenerate 
characters of other species of the same series. Of course ‘this ac- 
celeration affects both the ornamentation or ridges as well as the 
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