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three factors, viz., rapid spreading of the whorl, the abrupt curva- 

 ture and contact or close proximity of the paranepionic stage to the 

 apical part of the conch. Even, however, if this conclusion be 

 doubted and if, in a few forms of extremely specialized nautilian 

 shells of these early periods of geologic history, it can be asserted 

 that the impressed zone has really become inheritable ; the position 

 assumed in this paper, that the impressed zone is mechanically gen- 

 erated in the later stages of growth and becomes an inheritable 

 characteristic only in forms with accelerated development, is posi- 

 tively strengthened. The whole argument being based upon mor- 

 phology, it makes no essential difference how early the impressed 

 zone appears or in what form it appears, provided the shells in 

 which it is characteristic of the first volution before contact are the 

 descendants of those in which this character is transient and 

 obviously due to the moulding during growth of one volution over 

 the next inner volution. 



My experience, however, in writing this paper has led me to dis- 

 tinguish two kinds of impressed zones ; that which occurs on the 

 free dorsal sides of the young and that which occurs as the direct 

 result of contact. I propose therefore to call the former the dorsal 

 furrow and the latter the contact furrow. 



The ananeanic substage among Carboniferous cephalopods is not 

 only marked by the beginning of the contact furrow but also, as a 

 rule, by the introduction of correlative changes in the form of the 

 whorl. Thus the tetragonal whorl, with an outline similar to that 

 of an inverted trapezoid in section, and consequently an obvious 

 repetition of the ephebic whorl of Temnocheilus, and with sutures 

 also like those of the adults of that genus, appears at this stage in 

 Carboniferous cephalopods of several different genera, showing their 

 immediate descent from Devonian Temnocheili. 



The first appearance of the dorsal lobe in the sutures is correlated 

 with closer coiling and is apt to make its first appearance in primi- 

 tive nautilian shells at this stage in the contact furrow. This lobe 

 however, occurs also before the whorls touch in a number of forms, 

 notably Barrandeoceras of the Silurian, and in one of these, Bar- 

 randeoceras Sternbergi, it occurs in the ephebic stage, although 

 this is a gyroceran form and no contact furrow is formed. There 

 is also another smaller lobe which appears in the centre of this, 

 the annular lobe. These are not strictly correlative with the 

 impressed zone, since a dorsal lobe appears in some cyrtoceran 



