352 GEOLOGICAL BIOLOGY. 



2. The Helictitic type, in which the saddles (see ES, LS, 

 Fig. 1 14) is alone secondarily crimped. 



3. The Medlicottian type, in which the sides of the 

 saddles and lobes, .or lines connecting them, are dentate or 

 secondarily crimped (Fig. 119). 



To distinguish these three from the former type they may 

 be classified as the crenulated or secondarily crimped type. 



D. TJie Amino7iitic Type of Suture. — There is a still 

 higher complication of this system of sutures. The secondary 

 curvatures may be themselves tertiarily crimped or notched, 

 forming a tertiary system of lobation of the suture ; this gives 

 us the Ammonitic type of suture, and the suture is called 

 foliate to various degrees of elaboration in different genera. 



E. The Pinacoceran Type of Suture. — A further extreme 

 of differentiation is attained in the crimping of the edge of 

 the septum of Pinacoceras of the Trias (Keuper), of which 

 twenty-seven species are reported, in them the tertiary lobes 

 are again dentate or crimped, forming the quaternary system 

 of lobation. This is the highest stage of elaboration recorded 

 for the suture line of the Ammonoids (Fig. 117). 



Relation of Order of Succession of Initiation to Order of Ontoge- 

 netic Development and Evolutional History. — The natural law of 

 sequence of these various types of lobation of the suture is 

 that given above: (i) Nautilian, (2) Goniatitic, (3) Ceratitic, 

 (4) Ammonitic, (5) Pinacoceran, — that is, the order of succes- 

 sion is (ist) the simple, (2d) the lobed, (3d) the crenulate or 

 secondarily lobed, (4th) the foliate or tertiarily lobed, (5th) 

 the quaternarily lobed form of Pinacoceras, — and is so far an 

 arrangement of a series of related characters in normal pro- 

 gressive order. 



The question naturally forces itself upon us, What rela- 

 tion has this normal order of sequence of the characters to 

 ontogenetic development and to phylogenetic evolution? 



Order of the Ontogenetic Growth of these Characters. — i. 

 First, in ontogenetic growth (as illustrated in Fig. 116) we 

 find this order to be the order of sequence in the develop- 

 ment of the shell of an individual. The first, or protoconch, 

 stage has a Nautilian or simple suture, or what is the primi- 

 tive form of that suture (Fig. 116, G, and Fig. iii,^); the 



