part 1] ON THE MORPHOLOGY OF THE AMMONITE SEPTUM. 45 



It is interesting to notice that the asjanmetry due to the 

 abnormal characters of the hrst lateral lobe of the right side is 

 shown clearly in the septal sections as well as in the development 

 of the sutures, a fact which emphasizes the soundness of the con- 

 clusion j to which this comparison seems to be leading, that the 

 septal sections may be used for ascertaining some of the leading- 

 points in the develo])ment of the sutures. 



Septal section 9 (lig. 10 i) is comparable with the sutures of the 

 fourth .whorl (fig. 9/') except that the second auxiliary has made 

 its appearance, and that the frilling has clianged in the same way as 

 did the major folds of the earlier sections : that is to say, they haye 

 l)ecome deeper and narrower than in the corresponding sutures. 

 Their relative proportions are the same : for example, in the sutures 

 of the fourth whorl (9/') the external saddles are asymmetrically 

 subdivided. A similar asymmetry may be detected in the nintli 

 section (10/). The tenth section (10 7l) also bears much detailed 

 similarit}" to the sutures of the lifth whorl (9y) : observe, for 

 example, the five-fingered appearance of the left lateral lobe. By 

 this stage in development the animal has begun to assume adult 

 characters. 



XII. Septal Sections oe Tragophylloceeas loscombi 

 (J. Sowerby). 



The shape of the whorl of Dacfylioceras does not depart greatly 

 from that which is normal for ammonites. We have yet to ex- 

 amine to hovr great an extent the characteristic just observed in 

 Dactylioceras is true for ammonites in which the whorl has become 

 either compressed or depressed. As an example of a type with a 

 compressed whorl, TracfopliyUoceras loscombi is here taken. The 

 septal sections shown in lig. 13 (p. 46) are lettered for comparison 

 with the stages in the sutural development (fig. 12). The positions 

 of the sections in the whorl are indicated in fig. 14. 



The specimen of thife genus used by us was a young one. Its 

 size and last suture show that it comes in between k and / of 

 Mr. Spath's series (^op. cit. 1914, p. 340 ; see fig. 12). It shows 

 differences from his specimens, which must be borne in mind when 

 comparing septal sections made from it with the developmental 

 series given by him. For example, the great size of the terminal 

 leaflets of the external and first lateral saddles shown in his sutures 

 at h and 7 does not appear in the last suture of our specimen. 

 This fact may account for the relative smallness of those leaflets 

 in the septal sections. Again, Spath's figures of sutures h and / 

 do not form a continuous series with g and /' : compare, for 

 example, the second lateral saddle and also the terminal leaflets of 

 the external saddles. We have been unable to detect this break in 

 the earlier sutures of our specimen, so that its absence in the 

 septal sections can hardly l)e accepted as evidence against their 

 value. 



