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cal means, by the abrupt curvature of the whorl at the gyroceran 

 bend, and had not through time or constant repetition become 

 fixed in the organism and genetic, one ought to find in some species 

 of the Jura having larger umbilical perforations than others, that a 

 dorsal furrow was absent, or else variable and often very slightly de- 

 veloped. 



Suppose, on the other hand, without paying any attention to the 

 manner of the origin of the impressed zone, except in so far as the 

 facts show that it appeared late in the life of primitive species 

 and is an acquired character, one asserts that time and fixation in 

 nautilian shells has made it hereditary. 



It is then of no consequence whether a given shell of the Jura has 

 a large or small umbilical perforation. Being a highly specialized 

 nautilian shell and apparently without other than strictly nautilian 

 progenitors, it follows from the law of tachygenesis, that the im- 

 pressed zone ought to be represented by a dorsal furrow in the para- 

 nepionic substage, or earlier in every species. The mechanically 

 generated contact furrow of transitional nautilian shells occurs in 

 the ana- and metaneanic substages, rarely later, consequently if 

 the dorsal furrow arose out of this through the law of tachygenesis 

 it should appear in the preceding stages of the ontogeny before the 

 whorls touch in every shell of the Jura. 



It is of course possible that exceptions to this rigorous logical de- 

 duction might have occurred in diseased young individuals, or in 

 species directly traceable to arcuate forms in the Trias, but so far 

 no such shells have been found. 



In looking at the apices of the species of Digonioceras and of 

 Cenoceras, considerable difference is noticeable in the sizes of the 

 umbilical perforation. For example those of Digonioceras excava- 

 tum, PL xi, and Digonioceras, sp. (?), PL xii, Figs. 6-n, are com- 

 paratively quite large. But in these the dorsal furrow appears at the 

 same age as in Cenoceras intermedium and others having very much 

 smaller perforations and more rapid increase of the metanepionic 

 substage. In other words, the rapid increase of the ventro-dorsal 

 diameters and other diameters and the sudden bending of the shell 

 and the abrupt gyroceran curve of Ce?ioceras intermedium and line- 

 atum and clausum have no effect whatever upon the genesis of the 

 dorsal furrow. As if to make this conclusion still more secure, 

 Cenoceras aratus, the single species in the Jura, which does present 

 a slight acceleration in the development of the dorsal furrow, has 



