308 
and direct cause of the correlated changes of structure which dis- 
tinguish the different orders and suborders, and often of the 
exceptional genera and species. 
We will mention but one of these exceptional cases, in some 
respects the most pertinent—the existing Argonauta, or paper 
nautilus (Fig. 2, p. 357). Here a thin shell secreted by the mantle, 
by the edge of the mantle, and by the two pairs of long dorsal 
arms, encloses completely the animal of the female alone, the male 
being naked. As asexual organ for the protection of the eggs; as 
an adolescent and adult structure, originating at a late stage in the 
life of the individual, and not in the shell gland of the embryo; and 
in its microscopical structure—it is not a true shell, or similar to any 
true shell among Cephalopoda. Still, in form and position, and 
as built in part by the mantle, it is analogous to a true shell, and 
has in part also the functions of a true external shell, and ought 
therefore to support or refute the hypothesis maintained above. It 
belongs to a swimming animal, and should therefore have the 
hyponomic sinus in the aperture and striz of growth as in Nauti- 
loidea; and these it certainly has. Compare the side view of 
Nautilus umbilicatus (p. 354, Fig. 1), with the Argonauta and it will 
be seen that the lines of growth agree in both and that both pos- 
sess the hyponomic sinus on the outer side. One can appeal to 
this example as a most convincing exception to prove the rule that 
the shell is a true index of the most remarkable adaptive structures, 
and, among the fossils, can give us exact information of important 
similarities or differences in structure and habits. 
The efforts of the Orthoceratite to adapt itself fully to the 
requirements of a mixed habitat of swimming and crawling gave 
rise to the Nautiloidea; the efforts of the same type to become 
completely a littoral crawler evolved the Ammonoidea. The suc- 
cessive forms of the Belemnoidea arose in the same way. But here 
the ground-swimming habitat and complete fitness for that was the 
object. The Sepioidea, on the other hand, represent the highest 
aims as well as the highest attainments of the Cephalopods in their 
evolution into surface-swimming and rapacious forms. We cannot 
seriously imagine these changes to have resulted from intelligent 
effort ; but we can with Lamarck and Cope picture them as due to 
efforts on the part of the animal to take up new quarters in its en- 
vironment and thus acquire habits and structures suitable to the 
