166 me. s. s. bttooian on [June 1913, 



as they become involute and more aged * they lose inflation, and by 

 this criterion they may be distinguished from tbe preceding series 

 of genera. 



Now, when one series is travelling from thin to thick and another 

 from thick to thin, there is every possibility of their reaching a 

 median collision-point, thus : — 



Thin 



Thin Thick 



At the median point, given certain factors, there would be 

 homoeomorphy : not the homceomorphy of subparallel lines of de- 

 velopment as in the first-quoted genera, but the more interesting — 

 and, by the nature of the case rarer — phenomenon of homceomorphv 

 of crossing lines of development. The very interesting case of 

 Pacliyceras robustum and Cadoceras sublceve mentioned above (p. 16b) 

 is an example, and a very good example, of such homoeomorphy. 

 It caused incorrect identification and, in consequence, an un- 

 warranted extension of the zonal range of Cadoceras. 



Homceomorphy of the first kind may be called parallel 

 homceomorphy, that of the second transversal homoeo- 

 morphy: these terms refer to the manner in which the pheno- 

 menon was produced. Isochronous and heterochronous 

 homceomorphy state whether the homoeomorphous species lived 

 in the same or at different times. 



There is yet another mode of producing homoeomorphy, and this 

 may be called cyclical homoeomorphy. It arises when a 

 species in the course of its development retraces its tracks : that is 

 to say, when a species showing a catagenetic stage apes one in an 

 anagenetic stage. For example, an anagenetic species is in the 

 pre-tuberculate costate stage, the catagenetic species is in the 

 post-tuberculate costate stage. The position may be illustrated 

 diagrammatically thus : — 



tuberculate 



Tbe circles represent the main stages of the two species which 



1 There would be renewed evolution (outcoiling) in extreme old age (incipient 

 scaphiticone stage). 



