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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 

 Ammonitinae 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 upon 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 denned differently according to the position 

 of the animal in time and in the evolution of irs 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 material and study every detail of the development. 



