part 1] ox THE MOEtilOLOGr OF THE AMMONITE SEPTUM. 



37 



septum itself was 0*95 mm. broad, and 0'22 mm. from dorsmn to 

 venter. The extreme antero-posterior range from the crest of the 

 saddles to the trough of the lobes was 0"16 mm. The area shaded 

 in the ])lan is comparable with the flattened area of the adult, but 

 is relatively much less extensive. It lies between contours 0*03 mm. 

 apart, and occupies 30 per cent, of the area of the septum. On the 

 whole, the septum tends to be concave rather than convex forwards. 

 It is evident that the septum-secreting portion of the mantle, 

 Avhich was strongly convex at the time of formation of the proto- 

 eonch and even of the first septum, had already.begun to feel the 

 influence of that pressure, which in later life became strong 

 enough to impose upon it a marked concavity. In the more 

 primitive ammonoids, especially the Asellati,^ the form of the 

 earlier septa proves that this influence did not develop quite so 

 rapidl}' in them as it did in the specialized forms represented by 

 Dactylioceras, « 



(2) Comparison of the Second with the Adult Septum. 



On comparing the second septum of Dactylioceras (PI. Ill) with 

 the adult septum, it is seen that, apart from the peripheral zone, the 

 latter shows all the essential features of the former, so that, except 

 for the increase in size and flattening of the elements,' the major 

 portion of the septum has undergone no significant change during 



development. The 

 Fig. 7. — Graph illustrating the average 

 profile of the second septum o/ Dacty- 

 lioceras shown in PI. Ill, Jig. 4. 



.08- 



changes already 

 mentioned above : 

 namely, the relative 

 movements of the 

 first lateral saddle 

 as the height of the 

 whorl increases, and 

 the origin of the 

 new lobes and sad- 

 dles on the umbilical 

 wing of the septum, 

 are confined to the 

 marginal area. 



It must not be 

 supposed, however, 

 that this general 

 agreement of the major portion of the adult septum with the Avhole 

 of the second septum is due merely to a simple enlargement of the 

 area which secreted the latter. Had this been the case, the siph .ncle 

 would have retained the subcentral position of the second septum 

 throughout life. During the growth of the first and second whorls, 

 however, it gradually shifts to the ventral margin, and thus comes 

 to lie within the frilled peripheral zone. In some cases, according 



Datum -plane 



Area in front of successive contours. 



^ W. Branco, Palaeontographica, vol. xxvii (1880) p. 68. 



