592 
a great many forms in the paranepionic, is paralleled by the similar 
tendencies occurring in shells that have the whorls contiguous. 
This is the first effect of contact, and the formation of a lobe in 
the sutures also very commonly accompanies slight contacts. Nev- 
ertheless, dorsal lobes in the sutures and the flattening of the dorsal 
side may occur in cyrtoceran and gyroceran coils of species that 
appear to be transitional, from more primitive uncoiled to the close 
coiled nautilian forms, as in Barrandeoceras Sternbergt, Pl. xiv, 
and other examples, such as Aphetoceras boreale, Pl. v. 
These characteristics obviously exist under different conditions 
on the free whorls of primitive shells and the similar whorls of 
the young of nautilian shells than they do on whorls which are in 
contact. In order to make these distinctions clear, I have named 
the dorsal hollow zone that appears before or independently of con- 
tact, the dorsal furrow, and that which occurs after that, the contact 
furrow, both being considered part of the same feature, the com- 
pressed zone. 
Before proceeding further it is necessary to study the origin and 
history of the impressed zone, and to define it more clearly than 
has been done in the preceding pages. 
In the first place, as already stated, it does not exist in any of the 
trunk or radical forms, except Cranoceras. Its first appearance, so 
far as the morphology is concerned, is in nautilian forms after 
contact, and this occurs constantly in different genetic series. In 
fact the definition of a nautilian shell is based upon the possession 
of a contact furrow. 
If we regard any genetic series by itself we can often see that the 
impressed zone is purely a contact furrow. ‘Thus, in the Estonio- 
ceras, it is absent in the umbilical perforation on the dorsum of the 
nepionic stage and it is slight and present only in the contact stages, 
being soon lost upon the free part, or gerontic stage of the coil. In 
other species of some other groups the same thing occurs either com- 
pletely or partially: Eurystomites, Pl. v; Tarphyceras, Pl. vi; 
Schroederoceras, Pl. vii, and so on. 
In transitional species with large umbilical perforations, the dor- 
sal furrow is not present in any specimen, although many have 
been examined and recorded. In the major number of nautilian 
forms, in the Silurian, Devonian and Carboniferous and quite 
a number of Triassic species, the umbilical perforations are large 
and there are no dorsal furrows. In many of these species the 
