part 4] evolution or the lipakocekatid/i:. 251 



way related. It is, therefore, necessary to revise the generic 

 classification. 



Each genus, as has been shown, includes ammonites of very 

 different appearance, ranging from Capricorn e volute forms to 

 bituberculate involute forms ; in constructing the revised classifi- 

 cation offered here, the affinities of the various in embers of the 

 genera have been ascertained by the establishment of continuous 

 series of forms only separated by small differences attributable to 

 accelerated development. Further, the sutures of each of these 

 series have a number of features in common, and these constitute 

 the readiest means of identification. 



III. Stratigrapiitcal Notes. 



The time-ranges of these ammonite lineages are of importance 

 to the stratigraphical geologist, for the ammonites of this group 

 are dominant in the upper part of the Lower Lias, where it has 

 been usual to recognize a capricornus zone overlying a lienleyi or 

 striatus zone. But it is quite clear that the striatus-like forms 

 are descended from Capricorn ammonites, and such a conclusion 

 can only be reconciled with the facts of stratigraphy by supposing 

 that there were below the striatus zone capricorns which gave 

 rise to the involute forms of that horizon, and it is also probable 

 that involute forms have evolved from the capricorns of the capri- 

 cornus zone. Such capricorns of Tpre-striatus age, and involute 

 forms of ipost-cajyricornus age, will be described from Xapton 

 (Warwickshire) and Lincoln respectively. 



L T ntil late years it has been supposed that almost all the striatus- 

 like forms occurred in the so-called ' striatus zone.' Recently, 

 however, Dr. W. D. Lang l has carefully collected Liassic 

 ammonites in the cliffs of Dorset with reference to their precise 

 position. From a study of his specimens it appears that these 

 forms occur at several distinct horizons, as follows : — 



«j Involute ammonites (of the ? A. nauHUformis group), 

 zone. [ v J 



( Oistoceras J" Capricorn ammonites (Oistoceras) and involute ammon- 



sub-zone. \ ites similar to A. becliei. 



davceij Liparoceras t JnYolute ammonites similar to 4< }ien i eyL 



zone. I sub- zone. [_ 



a xcos a j Q a p r i corn ammonites of the A. latxeosta group. 

 ^ sub-zone. (_ 



("Capricorn ammonites of the Beaniceras series (see 



ibex zone. < p. 268). 



^ Involute ammonites of the Liparoceras series. 



It will be noticed that involute ammonites of the Liparoccras 

 series are only to be found in the upper part of the ibex zone, 



1 ' The Geology of the Charmouth Cliffs, &c. ' Proc. Geol. Assoc, vol. xxv 

 (1914) p. 293 ; 'The Ibex Zone at Charmouth, &c.' ibid. vol. xxviii (1917) 

 p. 31. 



