591 
In many shells with very large perforations the curvature is often 
uniform, and there is no sudden alteration in the direction of coil- 
ing of the first whorl, as in several Carboniferous forms and the 
remarkable shell Plewnautilus superbus, Pl. xii. The same is true 
of the coiling of all gyroceran forms in their ephebic and earlier 
stages. The coiling is more uniform than that of more special- 
ized and more closely coiled shells. This, and the presence of 
smaller umbilical perforations in the same genetic series, is easily 
accounted for if we admit that the tendency to become closer 
coiled is genetic, and that in accordance with the law of tachy- 
genesis it affects the growth of the young earlier in the more spe- 
cialized and later-occurring forms, thus shortening up the ana- and 
metanepionic substages. 
The elliptical outline in section, the universal rotundity of the 
dorsum in the ana- and metanepionic substages and the sutures, 
serve to reinforce the assumption that these substages derive their 
characteristics, so far as form is concerned, fromm arcuate or straight 
ancestors. This is in general the adult characters of most species 
of groups having orthoceran or cyrtoceran forms, and in none, 
except Cranoceras of the Devonian, has any signs of a dorsal fur- 
row been found. ‘The ornamentation of these substages, and usu- 
ally the paranepionic, in part or as a whole, also points distinctly 
to some straight or arcuate ancestor. The ananepionic substage is 
universally smooth or with only a few longitudinal ridges, but the 
metanepionic varies more. The form and markings in Vestinautilus, 
Pl. ix, point distinctly to a similarly ornamented arcuate ancestor, 
and the gradual shortening up of the younger substages is also 
shown by the figures on this plate and the explanations. 
The outline of the first volution changes abruptly at or imme- 
diately after the beginning of the gyroceran bend, that is, at the 
beginning of the paranepionic substage in most nautilian shells. 
The dorsum is apt to become flatter in species having large umbili- 
cal perforations, and in those with small perforations, this tendency 
is intensified and the dorsum is apt to become concave, the dorsal 
furrow making its appearance. ‘The sutures of the ana- and meta- 
nepionic substages are apt to have ventral and dorsal saddles, 
whereas a dorsal lobe very often appears in the paranepionic, ex., 
Barrandeoceras. This dorsal lobe is still more plainly marked when 
the dorsal furrow is present in the paranepionic volution. 
The flattening or broadening out of the dorsum, which occurs in 
