ILLUSTRATIONS OF ORTHOGENESIS 967 



accentuation of the lobes and saddles, and recapitulate the adult 

 sutures of such later Devonic types as Tornoceras and Prionoceras. 

 The fifth suture is transitional to the sixth, which is characterized 

 by a divided ventral lobe, one lateral lobe on each side, and another 

 on each side of the umbilical border. Here, then, the lobe which 

 complicated the original ventral saddle is itself divided by a second 

 low ventral saddle. The shell at this stage has a low, broad, in- 

 volute whorl, and in this and the character of the suture recalls the 

 adult of the older species of Glyphioceras. The typical Glyphio- 

 ceras condition is represented by a somewhat later suture, and still 

 later, with the appearance of a second lateral lobe next to the 

 umbilicus, the shell begins to resemble the late Carbonic goniatite 

 Gastrioceras, and at a still later stage, when the diameter of the 

 shell is 2.25 mm., a third lateral lobe appears next to the umbilicus 

 which subsequently widens, wdiile the whorls become higher and 

 narrower. In this stage it recalls the genus Paralegoceras. The 

 next stage ushers in true ammonitic ornamentation in the form of a 

 ventral keel (2.7 mm. diam.), while the suture still remains goni- 

 atite-like. But when the shell has reached a diameter of 3.2 mm. the 

 first lateral saddle becomes indented, a true ammonoid suture thus 

 coming into existence. The future development is along the line 

 of increasing complexity of suture. 



A consideration of the possible mutations which may come into 

 existence by the operation of the law of heterepistasis, or differ- 

 ential arrestation in development, and by the operation of the laws 

 of differential acceleration and retardation of characteristics, will 

 convince one that all the known types of ammonites, as well as 

 many yet unknown types, may be accounted for in this manner. 

 Not only all so-called species, but every individual variation will 

 fall into its proper determinate place in the series when the method 

 of analysis of individual characters has become sufficiently de- 

 tailed. 



Another example, taken from the gastropods, may serve to 

 further illustrate the principles here discussed (Fig. 256). The 

 modern Fulgur caricmn, a large gastropod occurring on the Atlantic 

 coast between Cape Cod and the Gulf of Mexico, begins its embry- 

 onic existence with a smooth shell drawn out anteriorly into a canal 

 and not unlike in form to some smooth Fasciolarian shells (Fig. 

 256, b, c). At a very early age the shell is furnished with ribs and 

 then an angulation appears in the outer whorl. On this angulation 

 the ribs are soon reduced to rounded tubercles. This condition 

 recalls the adult characters of Lower Miocenic species of this genus, 

 which never pass beyond the tubercled stage. This stage in the 



