35^ GEOLOGICAL BIOLOGY. 



id) In the typical Ammonite there is a tertiary crimping 

 of the suture-line, i.e., each of the archings of the line corre- 

 sponding to the crenulations of Medlicottia is again crenu- 

 lated, forming a complexly foliate suture. 



ic) In the adult forms of Pinacoccras there is a still further 

 elaboration of the crimping, the tertiary archings of the Am- 

 monite are again crenulated, forming a quaternary stage of 

 corrugation. 



The series presents a gradual elaboration of the crimping 

 of the edge of the septum, forming a suture line, ist, simple 

 26., primarily lobed, 3d, secondarily corrugated {\.\\t crenulated 

 type), 4th, tertiarily corrugated (the foliate type), and 5th, 

 with the quaternary corrugations of Pinacoceras. 



In their historical bearings it may be said of this series 

 that : 



1. It is the order in which the various types made their 

 first appearance in the geological series. 



2. It is the order in which the several types became 

 dominant. 



3. It is the order of elaboration in the ontogenetic growth 

 of the individual, 



4. It is the normal order of mechanical relation borne 

 by the several types to each other; each type is a mechanical 

 elaboration of the next preceding type. 



The convolutions of the suture are crimpings of the edge 

 of a more or less flat disk, — the septum, — and these convolu- 

 tions are the simplest mode of adjustment of the edge of 

 such a disk, whose circumference increases more rapidly than 

 its radius. 



Considering only the differences in the sutures, it would 

 be correct to state that if we assume that the one is derived 

 by modification from the other, it would be mechanically im- 

 possible for the Ammonite's septum and suture to be formed 

 without passing through the stages represented by the 

 Nautilus, Goniatites, and Ceratites. In other words, this 

 exhaustive analysis of this one element of structure of 

 cephalopod shells shows us that the actual history of these 

 organisms has been exactly that which a serial classification 

 on the basis of differences of this part would suggest, and 



