354 GEOLOGICAL BIOLOGY. 



veloped each, except the extreme Pinacoceran, stage of this 

 character. Immediately after, in the Trias, the Ammonitic 

 and Ceratitic types are both well developed and represented 

 by many genera. The historical order of initiation of the 

 several types of sutural modification is thoroughly consistent 

 with the order which an analysis of the nature of the modifica- 

 tions themselves suggests to be the natural order of sequence. 



When we examine the order of sequence of the stages of 

 dominance of the several types of suture the former conclu- 

 sions are also confirmed. The Nautilian, the Goniatitic, the 

 Ceratitic and its modifications, and the Ammonitic and its 

 modifications, became dominant in the normal order. And the 

 appearance of the extreme Pinacoceran type in the Trias, with 

 its failure ever to become dominant, is in keeping with the 

 general principle that it is rarely the case that extreme modi- 

 fications of a type are either longest to live or the best 

 adapted to struggle with competing types of organization. 



Rate of Elaboration of the Various Types of Suture. — 3. When 

 we look at still another relation of this series of facts, and 

 ask. What was the relative rate of expansion of this character 

 in comparison with the life-period of the race expressing the 

 modification ? we learn that regarding the character as origi- 

 nating in the straight Orthoceran form, the first stage of 

 sutural modification was reached when the first Goniatite 

 appeared ; this was near the base of the Devonian. The Cera- 

 titic and Ammonitic stages had both appeared before the close 

 of the Paleozoic, and by the early Trias the Pinacoceran had 

 appeared ; hence the extreme expansion of this character had 

 taken place between the base of Devonian and base of Trias, 

 but the life-period of this particular race of organisms reached 

 its close rather suddenly at the end of the Cretaceous; and 

 we may infer that the extreme limit of modification of this 

 particular character had been attained before the race ex- 

 pressing it had half finished its course. 



Rapidity of Modification of each Type soon after it was Initiated. 

 — 4. When we consider the degree and rapidity of develop- 

 ment in each of these types of suture-lines, we observe that 

 after the character had once appeared it was expressed in 

 numerous species and genera, and it expressed a tendency to 



