603 ~ 
form. It is to be anticipated of course in species of this kind that 
other characters will also show acceleration. Accordingly one finds 
as shown in several figures that in Co/. globatum a dorsal furrow 
is to,be found in the paranepionic substage. 
The umbilical perforation is of good size in this species, the 
curvature is often gradual and uniform, the ana- and metanepionic 
volution increases slowly in size, and there is apparently no 
mechanical agency in any of these characteristics that would have 
caused or led up to the appearance of the dorsal furrow in the para- 
nepionic substage. Another point is obvious in this species. It is 
a descendant of a special series which probably arose from Zhora- 
ceras Puzonianum and canaliculatum, or some species of more an- 
cient origin combining the characters of these two. ‘This series 
then obviously passed through the distinct phases of gyroceran and 
nautilian evolution and acquired a contact zone, which in the highly 
specialized phylogerontic Coloceras became by the law of tachy- 
genesis a dorsal furrow inherited in the paranepionic. 
The facts in my opinion cannot be accounted for on any other 
hypothesis. 
It is hardly doubtful when other involute and highly specialized 
shells have been fully investigated that many more examples of the 
accelerated inheritance of the impressed zone will be found. 
Nannoceras Frieslebent (P\. xi) is the only species in the 
Dyas that I have been able to investigate, and this has a dorsal 
furrow and a small umbilical perforation. Its congeneric forms are 
also unknown, and its evidence is consequently not of much value, 
except in so far as it shows the occurrence of this class of forms in 
this period. 
I was not able to obtain shells having small umbilical perforations 
and suitable for examination in the Trias, and have to leave that 
period a blank record except in so far as noted above. 
The close-coiled shells of the Jura are, however, sufficiently 
abundant and the evidence very interesting. 
In the first place, as noticed elsewhere, there are no arcuate radi- 
cals in existence. They have all disappeared in the Trias, and with 
them went also the transitional forms of all kinds, the gyroceran 
and even the primitive nautilian with very large umbilical perfora- 
tions. Under these circumstances one should expect to find a 
decided change in the behavior of characteristics. 
If the impressed zone was maintained and perpetuated by mechani- 
