part 1] OX THE MOEJ^HOLoar of the ammonite septum. 41 



111 the seventh suture (9 c) the same proportions are mamtainecl. 

 The new features to be noticed are the appearance of a median 

 ventral saddle, the increase in size of the lateral saddle, and the 

 presence of another saddle, the internal auxiliary, near the mn- 

 bilical angle. 



The folds just described persist throughout the remainder of the 

 first three whorls, Avith little change in their relative pro]Dortions. 

 Meanwhile crimping begins, and the Hrst external auxiliary appears 

 near the umbilical angle (9 <?). The external saddle becomes tri- 

 lobed, and thus resembles the internal saddle of the adult. The 

 first lateral lobe on the left is trifid, while that of the right is only 

 bifid ; thus, even at this early stage, the curious asymmetry already 

 noted in the adult has become established. 



In the fourth whorl (9^/) frilling becomes still more marked. 



In the fifth whorl (9y) crimping is carried farther, but its small 

 taxonomic value is made evident by a comparison of the right 

 and left sides. The left lateral lobe is wide, and has the primitive 

 five-fingered appearance of the corresponding lobe in Coeloceras 

 (fig. 8, iv & V, p. 39). Up to this stage the outlines are all 

 manifestly turgid. 



In the sixth whorl (fig. 9 h, p. 42) conditions giving rise to flaccid 

 ovitlines have begun to set in. 



The development of the dorsal series of lobes and saddles pro- 

 ceeds along similar lines to that of the ventral series, but the 

 details are later in appearing. For example, no signs of frilling 

 are present on the lobes and saddles of the dorsum at the fourth 

 ^^■horl (9/"), ^^et marked frilling was already manifest in the 

 ventral series before the end of the third whorl (9 e). Again, the 

 median saddle of the dorsum does not appear until the fifth Avhorl, 

 and it always remains small ; but the corresponding saddle of the 

 ventral series is present three whorls earlier. Further illustrations 

 of the same principle are afforded by the development of Trarjo- 

 phylloceras and 8pli(Erocera8. Branco noted a similar feature in 

 many other ammonites. The conditions in Clymenia (Palyeonto- 

 graphica, pt. 2, vol. xxvii, 1880, pi. viii) are exceptional, for the 

 dorsal folds are more deeply divided than the ventral. The fact 

 that in it the si2)huncle is dorsal suggests that the complexity of 

 folding is, in some way, associated with the position of the siphuncle. 



XI. Septal Sectiois^s of DACTVLiocEiiJS. 



In the foregoing pages it has been shown that the central area of 

 an adult septum is strikingly similar to the earlier septa. This fact 

 suggests that, if septal sections were made, they would show some 

 resemblance to the suture-lines at different stages in develo])ment. 



Fig. 10 (p. 43) shows a series of such sections made at the zonal 

 lines shown in fig. 11. Thus section I) represents the ground edge 

 of the septum at zonal line 2 of fig. 11 (p, 44). This illustrates 

 the extraordinaiy flatness of the innermost area of the septum. 



