622 



MOLLUSCA 



PHYLUM VI 



in their own development the ontogenetic stages of modification through 

 which the older ones have already passed. It follows also from this that the 



lobes and saddles nearest the umbilical lines of in- 

 volution are simple and often entire, and are parts 

 of a series that become progressively more compli- 

 cated outwards to the lines or columns of the 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 Beloceratinae, thus indicating unusual metabolism 

 on the venter like that of the regions of involution. 

 Jackson's law of the localised recapitulation of onto- 

 genetic stages is well exemplified by the history of 



Is AL 

 Fir;. 1148. 



Lytoceras fimhrlatum (Sowerby). 

 Middle Lias ; Wiirtemberg. Traiis- 



venTrarfote" "fLj^AntSiphonai s^^tures among Ammouoids as already shown by him 



lobe ; L, Siiperior lateral lobe ; in PlacenticevaS. 

 I, Inferior lateral lobe ; ES, Ex- 

 ternal saddle ; LS, Is, First and 

 second lateral saddles. 



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 Glyphioceratidae. Minor or marginal inflections are intro- 

 duced, and what are termed bifid or trifid lobes occur in the arms of the ventral 

 lobe (Fig. 1182) ; they then aff"ect the primitive first lateral lobes and saddles, 

 and extend thence toward the line of involution (Fig. 1187). 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 often 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. 



Siphwncle. — 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 

 in proportion to the volution, and apt to be nearer the centre (Figs. 1149, 1 150) 

 during the young than at later stages, and is also retrosiphonate, as in Nauti- 

 loids. It remains retrosiphonate in the Clymeniidae, Nautilinidae, and most 

 Gephyroceratidae,as well as primitive forms of Glyphioceratidae; but it becomes 

 transitional (having both funnels and forwardly directed collars) in more 

 specialised Carbonifei'ous Glyphioceratidae, and finally prosiphonate (funnels 

 lost, collars alone remaining) in Permian genera. Most Triassic and all Jurassic 

 and Cretaceous genera have the siphuncle chloiochoanitic. The above stages 



