﻿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 Sternberg}, PI. 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 



