﻿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 (PI. 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, is in 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, PI. xi. There are 

 several other species in the Trias that belong in the same category, 

 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 Vestinautilus konincki. 

 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 



