ORDER ii AMMONOIDEA ., I 1 



discriminated ;iml described, but other\vi>- are li;illc to confuse the nomen- 

 clature. Adventitious inflections (Fig. 1098) arise between tin- 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 tir>t 

 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 line- m 

 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 

 oritogenetic stages of modification through which the |.,, 



older ones have already passed. It follows also from ivtoeerae tmtviatum, s,,wi,. sp. 

 this that the lobes and saddles nearest the umbilical 2^^*5*3! 

 lines of involution are simple and often entire, and ^be^ VIS ^rio'r ui 1P ioS? 

 are parts of a series that become progressively more i, inferior lateral lobe ; ;>, K\- 



T j A.I. v i r xi. ternal saddle; / 



complicated outwards to the lines or columns ot the second lateral saddles, 

 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. 



Siphunde. 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 



