w \.,r\ 



There ia ■ similai »n in the evolution of the varietal and specific 



characteristics of the series. Thus, in I' II . I r lit. 11, and 16 repre- 

 aent tin' extreme varieties oi I '. the former with immature pilaa 



without channels, tin- second with well developed pilaa without channels, and 

 tin" third with well developed pilaa and distinct channels. 1 The young of Am. 

 i '. shows thai the derivation of this species was probably 



from the unchannelled forms of Am. tenucottaium similar to Fig 1 1 The 

 same i» true of 1 B with reference to Bartmeuwi, as is Bhown by com- 



: tin- young of tin- former, Plate 11. Fig. 23, with tin' adull of // 



i 7. which bad very slight channels even in the adult, and this i< still more 



apparent in the involute flattened form of Fig 24, in whicb tin- channels 



earlier developed. The mnection of all its characters, both of 



g and lull grown, with it* immediate ancestor, forbids us imagining an 



independent descent from :i variety of any other species than // ri, ami 



evidence is Btrong that i' bad no descendants beyond it- own Bpecies. 



The channelless variety of Am. fakaries, Fig. -<■ nilar evidence of its 



inection with the channelless varieties ol Am. Ha • or if this is 



more direct connection with I may he claimed; hut 



certainly there i- no evidence for any connection with tin- channelled varieties 



/.' 



in with psiloceran-like immature folds, after the keel appeared 



in th«- arnioceran varieties of Car. kridion, and before the keel in the i •>• acceler- 



levelopment of other varieties of tin- same Bpecies. In Borne forma the 



completed ami became tuberculated before the channels appeared. 1 



i 1 to tin- time ol' the appearance of characteristics 



with relation to each other i- a peculiarity of highly accelerated Bpecies, aa 



Such nee, Bince they enable ua to understand that 



do not necessarily develop with invariable regularity. Th< 



may be in a measure changed, or even reversed, when 

 one character more than another. So far we have 

 found ■ onlj in Bpecies where all tin- principal characters "i the 



ng exceptional a< leration. 



The common form of the younger stages of all the Ammonitinsa during the 



shown in the plates The depressed goniatitic helmet Bhape 



led in Psiloceraa bj a laterally flattened helmet -hap. I I 



me form v led tor ;i \<r\ prolonged period, in Bpecies with flattened 



abdomens, like ('"/. ■ •• in which the abdomen became broader 



and the sides slightly <1. ta in Cai 



in lull eai -I. replacing entirely the psiloceran helmet 



I 



