418 
while the shell is still in the nepionic stage and as it approaches the 
point of departure from the spiral and the subsequent loss of the 
contact furrow. Dr. Brown records that the spacing of the septa 
increases after the deposition of the twelfth septum, and that these 
partitions are more widely separated. This correlates with a cor- 
responding increase in the lateral diameters and together indi- 
cate an increased rate of growth. Nevertheless there is no quicken- 
ing in the processes of development nor any resumption of pro- 
gressive characters. The shell becomes a compressed ellipse in 
section, loses the contact furrow, and the straightened cone does 
not acquire the digitate sutures and pass into the neanic stage of the 
Ammonitinze until after it has departed from the spiral.* 
It is clear from this and other examples taken from later stages of 
growth that these are tachygenetic forms so far as the early inherit- 
ance of gerontic characters is concerned. Correlating with this, 
or in consequence of this, the inheritance of progressive characters 
in the sutures is delayed, and these parts change more slowly in 
these phyloparaplastic shells than in the phylometaplastic forms of 
the same order. The internal structures and the shell itself also, as 
previously stated, never attains even in the stage of ephebic devel- 
opment characteristics comparable to those of phylometaplastic 
species. 
It follows upen the preceding remarks that the characters of these 
stages have different duration in different members of the same 
genetic series, being more prolonged in the more primitive and 
shortened up through the action of tachygenesis in the more special- 
ized shells of the same series. It is also obvious that the limits of 
each substage must be defined differently according to the position 
of the animal in time and in the evolution of its own special series. 
There are theoretically no exceptions to this law in its broadest 
acceptation, but in its practical applications this is not the case. 
Thus the protoconchial stage is so nearly invariable in each order 
that it is characteristic of all Nautiloidea and all Ammonoidea, having 
peculiar characters in each of these orders, but this comparative in- 
variability is less apparent in the characters of the ananepionic, 
metanepionic and paranepionic substages, and especially in the 
neanic stage, which are not as constant. The tendency to change 
* Having received specimens of these precious fossils through the kindness of Dr. 
Brown, I am able to confirm his observations, although I have not yet had proper oppor- 
tunity to go over all the materval and study every detail of the deyelopfnent. 
