﻿543 



The dorsum becomes flattened in the latter part of the metane- 

 pionic substage and other transitions to the nephritic outline are 

 obvious in the gradual spreading out of the transverse diameters. 



The dorsal furrow appears as described above sometimes when 

 the bend is abrupt and sometimes when it is gradual ; in other 

 words, it is obviously not correlated with the size or shape of the 

 umbilical perforation nor dependent upon the curvature of the 

 volution. It appears always in the same place at or about the third 

 suture and when the nephritic outline is assumed at the beginning 

 of the paranepionic substage. But it will be observed in section 

 Fig. 4, that the outline, which has been very carefully drawn, is 

 not remarkable for being very broad in proportion, nor does the 

 study of this specimen give any grounds for supposing that the dor- 

 sal furrow could be considered a necessary condition of the mode 

 of growth. The curvature is about the same during the remainder 

 of the first volution, but the zone broadens with growth and devel- 

 opment of the nephritic outline, as may be seen in Figs. 2, 11, 13, 

 14 and Sec. 3. This zone has longitudinal ridges, but these are 

 much finer than those of the sides and abdomen. 



The neanic stage begins when the longitudinal ridges and cen- 

 tral zone disappear on the venter. The ridges persist on the 

 dorsal side, but disappear in what is probably the paraneanic sub- 

 stage, leaving the heavy lateral ridge and its accompanying flutes. 

 The neanic stage is therefore phyloanagerontic. 



The ephebic stage is perfectly smooth and phyloparagerontic in 

 aspect. 



The action of tachygenesis upon degenerative characters is thus 

 clearly apparent throughout the neanic and ephebic stages in this 

 interesting species. This fact is entirely in accord with the princi- 

 ples of Bioplastology as explained above with regard to the action 

 of this law upon retrogressive characters.* 



RineceratidcE. 



The figures of PI. ix show that this family has characteristics 

 closely resembling the arcuate forms of Thoracoceras which are 

 repeated in the ananepionic substage. Rineceras, however, never 

 has a hollow ventral central zone but remains gibbous on the abdo- 

 men throughout life. 



This characteristic also serves to distinguish the nautilian mem- 

 * See pp. 373,415,417. 



