part 1] ON THE MOEPHOLOGY OF THE AMMO^^ITE SEPTUM . 



89 



some change — which, following the prevailing custom, may be 

 ascribed to the onset of senile decay— had taken place in the 

 animal. This expressed itself in the "^suture -line by a considerable 

 decrease in the antero-posterior range of the lobes and saddles, 

 accompanied by a swinging forward of the umbilical portion of the 



sutm-e-line. These 



Fig. H.—Sitture-li)ie of Dactylioceras changes, which lead 



and Cwloceras. ^ to a crumbling-down 



of the apices of the 

 saddles to approxi- 

 mately the same plane, 

 strongly suggest a 

 diminution in the 

 forward convexity of 

 the septum. Mean- 

 while the saddles 

 broadened out, and 

 the demarcation from 

 the lobes became more 

 ill-defined. In minor 

 details the later sep- 

 tum seems at first more 

 intricately wrinkled, a 

 feature which is still 

 better exhibited in 

 ^y\r^^ ' \ / \ ( A other specimens (fig. 



*■■ " " " ^ J . 8, i), some of which 



show these and other 

 * ageing ' characters 

 through a wide range 

 of septa. This com- 

 plexity is strongly 

 suggestive of the 

 wrinkling of a collaps- 

 ing or flaccid bladder, 

 as opposed to the 

 simpler and more tur- 

 gid outlines of the 

 folioles in earlier 

 septa, and suggests a 

 diminution in the vigour of gas-secretion in the declining period 

 of life. 



The features that distinguish the earlier septum become more 

 obvious in the primitive members of the family, and lead by an 

 almost perfect gradation of changes to the condition observed in 

 Coeloceras pettos (fig. 8, v). 



The Dactyliocerates are evidently a decadent offshoot of 

 Coeloceras.^ In them are to be seen the phenomena which 



^ S. S. Buckman, ' On the Grouping of some Divisions of so-called " Jurassic " 

 Time ' Q. J. G. S. vol. liv (1898) p. 442. 



\l-Z — Bactylioceras commune ; 2, at the diameter 

 of 45 mm. ; 3, at the diameter of 39 mm. 10 

 sutures earlier in the same specimen. 4 = 

 Cceloceras fonticulus from Whitby ; diameter, 

 28 mm. 5^ Cceloceras pettos from Baden; 

 diameter, 27 mm.] 



