part 1] THE MOEPHOLOGY OF THE AM^EOXTTE SEPTTTM. 49 



Nevertheless, the similarity is sufficiently striking to justify the 

 opinion that the principle educed from the septa of normally-shaped 

 and compressed whorls holds good also, with the limitations noticed, 

 for forms with depressed whorls. 



Fig. 17. — Section of the whorl of Sphseroceras hrongniarti along 

 the datum-plane of the septum of which septal sections are 

 shown in fig. 16 (p. 48). The 'lines parallel to the peri- 

 pherif shoic the positions of these sections. 



XIV. The GrEXEKA.!. COXSIHERATIOX OF SePTAL SeCTTOXS. 



Mr. E. T. Jackson ^ has drawn attention to the phenomenon 

 of localized stages in development : that is to say, during life 



' stages may be foimd in localized parts which are similar to stages found in 

 the young, and the equivalents of which are to be sought in adults of ancestral 

 groups.' 



Thus, for example, the auxiliary saddles in the suture of Trago- 

 j)hylloceras are similar to the external and lateral saddles of the 

 first suture. The idea has been applied to the study of different 

 species in a progressive series {Lytoceras) by" Mr. S. S. Buckman,- 

 who has shoAvn that the first lateral saddle of an early tyjje is 

 at the same stage as tlie second lateral of a more advanced type. 



The similarity of the central area of the adult septum to the 

 earlier septa is not, however, an illustration of the same prnici]:)le. 

 All Jackson's illustrations are drawn from newly developed parts, 



1 ' Localized Stages in Development in Plants & Animals' Mem. Boston Soc. 

 Nat. Hist. vol. v (1899) p. 92. 



- ' On certain Genera «fc Species of Lytoceratidae ' Q. J. G. S. vol. Ixi 

 (1905) p. 151. 



Q. J. G. S. No. 289. - e 



