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dea, wherever they occur, whether in the Silurian or in the Creta- 

 ceous, invariably have coiled young, showing that they were the 

 offspring of coiled or nautilian shells, that is, of progressive forms 

 which have themselves been evolved from a series of straight arcu- 

 ate and gyroceran predecessors. Their uncoiling is a truly retro- 

 gressive character, and this tendency is inherited in successive 

 forms in several series, and thus the whole structure is finally 

 affected, the whorl reduced in size, and the complication of the 

 sutures and shells at all stages of growth is degraded until, in the 

 development of the individual, only the close-coiled young remain 

 to testify to their exalted ancestry. In other words, the forms really 

 inherit degraded characteristics at such an early stage that it affects 

 their whole life except the earlier stages.* 



If we examine any of the progressive series we find that charac- 

 teristic modifications or variations tend to appear first in the later 

 stages of growth and, as a rule, in adults, then in successive forms 

 of the same genetic series they tend to appear at earlier stages of 

 the ontogony and finally often disappear altogether or become 

 embryonic, and this is the case also with the degraded characteris- 

 tics. This is clearly shown in the illustrations given on Pis. ii, iii, 

 iv, especially in the history of the development of the sutures 

 among Ammonitinse. The simpler sutures of the Nautilinidae of 

 the Silurian and Devonian have undivided ventral lobes and broad 

 lateral lobes. The more specialized forms of the same suborder in 

 the Devonian have the ventral lobes divided, prominent saddles are 

 also introduced, and the lateral sutures become more sinuous. 

 These characters, especially the division of the ventral lobes, occur 

 in these forms (as in Fig. 17, PL 2) in an early neanic substage, 

 having replaced the hereditary undivided ventral of the adults of 

 the Nautilinidae and forced this characteristic back until it is 

 repeated only in the earlier or paranepionic sutures. In the Am- 

 monitinse of the Trias and Jura this process is carried still farther. 

 The repetition of the undivided ventral of the Nautilinidae is con- 

 fined to the earlier septa, which show sinuous lateral outlines (as in 

 Figs. 2, 3, PL 4) and these septa become immediately convex, the 



* Several examples are given of such forms among Xautiloidea in the text and the 

 similar uncoiling of the gerontic or senile stage is shown in the ontogony of a number of 

 species in the plates, notably Eurystomites kelloggi (PI. iv. Fig. 1). Among Ammonitinse 

 see young of Orioc. studeri (PI. iii, Figs. 11, 12). Crioc. studeri, after Barrande (PL ii, Fig. 

 40), Ancyloc. calloviense, after Barrande (PL ii, Fig. 41), and Baculites. after Brown (PI. 

 iii, Fig. 13). 



