MORPHOLOGICAL D IFFERENLLAl^ION. 339 



The characters which serve most readily to distinguish the 

 Ammonoids from the Nautiloids are the sutures. In the 

 Nautiloids, above described, the suture is always straight, or 

 but slightly curved. In all the Ammonoids the suture is 

 more or less lobed or notched. 



Mode of Curvature of the Nautiloid Shell. — Third. Before 

 considering the Ammonoids, we may notice the law of varia- 

 tion expressed by this curvature of the shell. In the Nau- 

 tiloids there are four types of form expressed in the direction 

 of growth of the cone : 



1. The shell is straight, or nearly so (see Orthoceras, 

 Fig. no, F). 



2. The shell is simply arched (see Cyrtoceras, H, /, D). 



3. The shell is discoidal, rolled in a spiral in single plane 



{a) This spiral may be loose {C ox J, Gyroceran) ; 

 {U) or close-coiled, (Goniatites, M) ; 

 {c) or involute (Nautilus, B). 



4. The shell may be spirally coiled in a screw plane, or 

 helicoidal. (See Fig. 108, p. 332). 



When we separate out for special consideration the mode 

 and amount of curvature of the shell, we arc first struck with 

 the evidence of progressive modification from the straight to 

 the close-coiled forms; but when the relation of these modi- 

 fications to the time of their first appearance is noted we learn 

 that forms of the different types of modification occur at the 

 earliest period (the Ordovician) in which the suborder appears, 

 and if we were to seek a series representing gradual modifica- 

 tion from one extreme to the other, we could find them well 

 expressed at this initial stage of the subordinal history. 



The Rate of Initiation of the Orthoceratidae. — For instance, 

 take the species of America alone, and of the straight or 

 slightly bent form, Orthoceratidae, there are recorded by 

 Miller 5 recognized genera and 170 species in the first stage 

 of this family, Ordovician ; in the Silurian there were 3 genera 

 and 81 species; in the Devonian 3 genera with 145 species; 

 in the Carboniferous, 2 genera with 43 species. 



Rate of Initiation of the Cyrtoceratidae. — The same general 

 law is seen in the (2) arched forms included in the family 



