415 
of the Trias, it is apparently shorter in duration than in the gener- 
alized and less complex organization of Tirolites if one can judge 
by the simple characters of the ephebic stage. 
In the Jura and Cretaceous, among the Ammonitinz and Lyto. 
ceratin, typical Ammonoids with more highly specialized struc- 
tures than any Triassic shells, the primitive characters of this sub- 
stage are, as one can read in Branco’s drawings and to a less extent 
in mine, still more limited in extent, being confined as a rule to a 
few sutures or to one, and finally, in many forms they are obliter- 
ated altogether. That is to say, the divided ventral lobe encroaches 
upon and finally obliterates the intermediate stage so that the meta- 
nepionic substage, which begins with the third septum and micro- 
siphon, is wholly changed in the aspect of the sutures. In other 
words, the undivided ventral lobe of the Nautilinidz has been re- 
placed in this substage by the divided ventral of the Primordialidz 
which appears in the suture of the second septum. 
This is also, like the preceding, an excellent example of what is 
meant by the law of tachygenesis, the earlier inheritance. through 
the crowding back and replacement of distal by proximal genetic 
characteristics. 
Fig. 3, Pl. iv, shows the prolonged duration of the nautilinian 
characteristics in this substage in second, third and fourth septa of 
Vermiceras (Arietites) spiratisstmun of the Lower Lias, the decided 
change to a divided ventral and two lateral lobes not coming in 
until the seventh suture. 
Fig. 7, Pl. iii, shows the section of Deroceras planicosta of the 
Lower Lias and the delayed approximation of the siphuncle to the 
ventral side. Fig. 7 shows the primitive structure of this organ in 
the earlier substages, and the figures from Branco show the duration 
of characteristics to be in correlation-with these primitive charac- 
teristics. 
Fig. 7, Pl. ii, shows the structure of the siphuncle in the meta- 
nepionic substage. The transitional aspect of the second septum 
can be observed in Figs. 6 and 7 of the same plate. This isa 
direct reference, as I shall show in another -paper, to the similar 
structure of the ephebic siphuncle, and also the swollen aspect of 
the early stages of the siphuncle in the Endoceratide, although in 
some species of this family as many as six funnels may take part in 
the construction of the swollen apical end of this organ. ‘These 
facts are also in direct correlation with the more specialized and 
