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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 Col. globatum a dorsal furrow 

 is to be found in the paranepioiiic 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 Thora- 

 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 Frieslebeni (PL 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 median i- 



