372 
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, asa 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 Pls. ii, 111, 
iv, especially in the history of the development of the sutures 
among Ammonitine. The simpler sutures of the Nautilinide 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 Nautilinide and forced this characteristic back until it is 
repeated only in the earlier or paranepionic sutures. In the Am- 
monitinee of the Trias and Jura this process is carried still farther. 
The repetition of the undivided ventral of the Nautilinidze 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 Nautiloidea 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 (Pl. iv, Fig. 1). Among Ammonitine 
see young of Crioc. studeri (Pl. iii, Figs. 11, 12), Crioc. studeri, after Barrande (Pl. ii, Fig. 
40), Ancyloc. calloviense, after Barrande (PI. ii, Fig. 41), and Baculites, after Brown (Pl. 
iii, Fig. 18). 
