604 
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 Digontoceras, sp. (2), Pl. xii, Figs. 6-11, are com- 
paratively quite large. But in these the dorsal furrow appears at the 
same age as in Cenoccras 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 Cenoceras 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 
