— 



whorls of the more advanced 

 members are not an indication of 

 different origin from the ammon- 

 ites which pass through a Capricorn 

 stage ; this skipping of the capri- 

 Jj, \. * ^ ^^ corn stage during development 



(saltative palingenesis) ] being 



' a compromise clue to the necessity of 

 attaining as perfect a condition as 

 possible in as short a time as possible, 

 for the sooner the adult condition is 

 reached the sooner will the organism 

 be able to reproduce its kind." - 



Owing to the method of origin 

 of the tubercles as points on the 

 ribs, they are at first necessarily 

 arranged in pairs. When the 

 whorl becomes swollen, however, 

 the line of outer tubercles is much 

 longer than the line of inner tu- 

 bercles ; in many series the number 

 of outer tubercles progressively 

 increases, until it may be double 

 the number of the inner tubercles. 



From this stage, therefore, two 

 distinct groups of Liparoceratid 

 ammonites may be recognized, 

 namely: — 



(1) An earlier group with tubercles 

 paired in the globose stages 

 (parinode forms). 



(2) A later group with tubercles un- 

 paired in the globose stages 

 (imparinode forms). 



Almost all British Liparocera- 

 tids belong to the second group 3 ; 

 Quenstedt's figures show, however 

 (4, pi. xxviii, (igs. 16-23), that 

 ammonites of the first group (A. 

 striatus parinodus} are of fairlv 

 common occurrence on the Conti- 

 nent. In considering the second 

 group it is often convenient to ex- 

 press the ratio of outer to inner 

 tubercles as a fraction, the ratio 



1 Since this paper was read Dr. W. D. Lang has suggested the term 

 'lipogenesis ' in place of saltative palingenesis (Proc. Geol. Assoc, vol. xxx, 

 1919, p. 60). 



2 A. Dendy, ' Outlines of Evolutionary Biology ' 2nd ed. (1912) p. 273. 



3 I have examined only nine British Liparoceratids in the splnerocone stage 

 with paired tubercles ; these were collected at a low horizon at Radstock, ' 

 Somerset (see p. 264). 



