part 4] evolutiox of the liparocekatidj:. 257 



There are, therefore, five progressive characters which inay be 

 used to determine the position of an}'- specimen in its series, 

 namely : — 



(1) Ornamentation, progressing from smooth, and costate, to bitubercu- 



late (paired) and possibly bituberculate (unpaired). 



(2) Whorl shape. 



(3) Involution. 



(4) Sutural complexity. 



(5) Development of longitudinal striations. 



Certain of these characters are acquired at a relatively earlier 

 stage in some series than in others ; thus the sutures of the capri- 

 corns of Androy y noceras and Liparoceras are similar in degree of 

 complexity, yet in the corresponding later forms the sutures of 

 Liparoceras are much more complicated than those of specimens 

 of And roqy noceras showing similar involution and tubercle-ratio. 

 The differing associations of these characters will, therefore, be. 

 useful in determining the various lineages. 



V. Systematic Study of the Gtenera. 



While there are many features in the adult sutures which are- 

 common to all the genera, there are nevertheless some readily- 

 distinguished sutural characters which make their identification 

 easy. Once the genus is accurately determined the species within 

 each genus are readily identified, for there is naturally much 

 greater outward similarity between the Capricorn s of two series 

 than between a Capricorn and its later development in the same 

 series — or, in other words, the chief difficulty lies in the distinction 

 of morphic equivalents. 



The sutures of the ammonites studied agree in having a deep 

 external lobe, divided by a median saddle which is usually very 

 high and narrow ; external saddles high, often divided into three 

 very unequal folioles by two denticulations, that of the dorsal side 

 frequently cutting deeply into the saddle. The first lateral lobe is 

 generally terminated by three lobules, sometimes asymmetrical. 

 The first lateral saddle is narrow, and always smaller than the 

 external saddle ; the relatively simple auxiliaries are of variable 

 number. 



The most useful characters in classification are : 



(1) The relative depths of the external and first lateral lobes. 



(2) The relative widths of the external saddle and of the first lateral lobe. 



The sutures of the chief genera are shown diagrammaticallv 



fig. 1 (p. 258). 



It is found that species of Liparoceras have a wide first lateral 

 lobe and a narrow external saddle, the first lateral lobe and the 

 external lobe being about equal in depth (tig. la); the species 

 which are at present referred to the genus Liparoceras all show 

 these proportions. In addition A. heterogenes, which was formerly 

 placed with Androyi/noceras (1, vol. i, no. 4(3), evidently belongs 



m 



