350 GEOLOGICAL BIOLOGY. 



Serrati (= Irregulares Beyr.), (5) Crenati (= Primordiales 

 Beyr.), (6) Acutolaterales, (7) Magnosellares (= Simplices 

 Beyr.), (8) Nautilini (= Nautilini Beyr.). (See Fig. 113.) 



Hyatt distributed the Goniatites into several families, 

 including in each the several groups based on sutural charac- 

 ters as follows: (i) Nautilinidze (Nautilini Beyr.), (2) Primor- 

 dialidae (Primordiales Beyr., and Crenati Sandb.), (3) Magno- 

 sellaridae (Magnosellares Sandb., Acutolaterales Sandb., Sim- 

 plices Beyr. ,/./., and ^quales Beyr.), (4) Glyphioceratidse 

 (Carbonarii Beyr., Simplices Beyr., /./., Genufracti Sandb., 

 Indivisi Bronn), (5) Prolecanitidffi (Lanceolati, Linguati, 

 Serrati Sandb., Irregulares Beyr,). (Hyatt included here the 

 genera Medlicottia, Sageceras, and Lobites, referred to the 

 Ammonites by Zittel.) 



Differences in the Sutures of the Ammonoidea explained as 

 Various Degrees of Crimping of the Edge of the Diaphragms. — 

 The sutures may be considered as simply the edges of the 

 diaphragm which is built by the animal across the conical 

 shell in which it lives, to constitute air-chambers of the va- 

 cated part as the animal grows in size. A simple explanation 

 is suggested by the mechanical principle that the natural result 

 of attempting to force a diaphragm into a tube too small for it 

 would be the crimping of the edges of the diaphragm. With 

 this clue applied to the interpretation of the sutures, we dis- 

 cover that all the various sutures may be defined in terms of 

 difference in degree of complexity of the crimping of the edge 

 of the septum. 



Classification of the Types of Sutures. — Gathering statistics of 

 all the known forms, and studying their embryological devel- 

 opment as well as their actual differences, we find the follow- 

 ing facts to be true regarding the modifications of the suture- 

 lines which result from the crimping or fluting of the outer 

 margin of the septum where it is attached to the wall of 

 the chamber of the shell : 



A. The Naiitilian Type of Suture. — In the Nautilidae the 

 suture is simple, either straight or slightly curved, but never 

 folded, i.e., in its complete circumference not exceeding a 

 single oscillation of curvature (see Fig. 102). This is the 

 Nautilian or simple type of suture. 



