ORDER II AMMONOIDEA 541 
discriminated and described, but otherwise are liable to confuse the nomen- 
clature. Adventitious inflections (Fig. 1098) arise between the first pair 
of laterals and the median line of the venter, either 
by the growth of marginals in the arms of the ventral 
lobe, or by division of the outer parts of the first 
lateral saddles, or by division of the inner parts of 
the siphonal saddle. 
The regions of greatest metabolism or growth- 
changes in each genetic series are near the lines of 
involution, and it is here that new inflections are 
usually formed. The later formed lobes and saddles 
in these regions repeat in their own development the 
ontogenetic stages of modification through which the Fic. 1099. 
older ones have already passed. It follows also from _ Lytoceras jfimbriatwm, Sowb. sp. 
this that the lobes and saddles nearest the umbilical S025: Se 
lines of involution are simple and often entire, and Ventral lobe; AZ, Antisiphonal 
% E lobe; L, Superior lateral lobe; 
are parts of a series that become progressively more 1, Inferior lateral lobe; ES, Ex- 
complicated outwards to the lines or columns of the ete 
oldest class—the first lateral lobes and_ saddles. 
When there are adventitious lobes, this series is reversed on the ventral side 
of the first pair of saddles. The inversion is sometimes quite complete, as 
in some of the Glossocampyli, thus indicating unusual metabolism on the venter 
like that of the regions of involution. Jackson’s law of the localised re- 
capitulation of ontogenetic stages is well exemplified by the history of sutures 
among Ammonoids as already shown by him in Placenticeras. 
The above method of designating the lobes and saddles as paired in the 
external aspect and on the dorsum on either side of the mesal plane disregards, 
for sake of convenience, an important fact that should be noted ; namely, that 
the azygous ventral and dorsal lobes are in reality paired with each other in 
the mesal plane; also that the primitive dorsals and external lateral inflections 
correspond in the same sense to one another, and are also more or less united 
across the septa in some forms. 
The outlines of the paired lobes and saddles first become complicated in 
the Carboniferous Eurycampyli. Minor or marginal inflections are introduced, 
and what are termed bifid or trifid lobes occur in the arms of the ventral 
(Fig. 1156) lobe; they then affect the primitive first lateral lobes and saddles, 
and extend thence toward the line of involution (Fig. 1159). These marginal 
inflections increase greatly in number and complexity during the Permian, 
become preponderant in the Trias, and universal in the Jura and Cretaceous. 
During the Carboniferous it is the lobes only, as a rule, that are thus modified ; 
but in the Permian the saddles too are generally affected. The modifications 
in outline proceed from the lobes to their sides, and thence to the saddle bases, 
except in certain cases when direct division of the saddles takes place by the 
outgrowth of secondary median lobes that divide their bases. All these 
secondary lobes and saddles are termed marginals. 
Siphuncle-—The caecal condition of the siphuncle is apparently confined to, 
the ananepionic stage or first septum, but J. P. Smith has shown that some 
species of Lytoceras and Phylloceras have a bulbous enlargement of this organ, 
which may persist in several nepionic camerae. This is apparently a persistent 
remnant of the caecal enlargement. The siphuncle of all Ammonoids is larger 

