FEBRUARY 1, 1884.] 
exceptional forms, — such as the Spirula 
among Belemnoidea, and among Sepioidea the 
octopods ; and we think they all prove our po- 
sition, that the habitat so closely accords with 
the structural changes of the type that its 
purely physical agency must be regarded as 
the efficient and direct cause of the correlated 
changes of structure which distinguish the 
different orders and sub-orders, and often of 
the exceptional genera and species. We will 
mention but one of these exceptional cases, in 
some respects the most pertinent, — the exist- 
ing Argonauta, or paper nautilus. Here a thin 
shell secreted by the mantle, by the edge of 
the mantle, and by the two pairs of long dor- 
sal arms, encloses completely the animal of the 
female alone, the male being naked. As a 
sexual 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 
notin 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 a homologue of a true 
shell, and has in part, also, the functions of a 
true external shell, and ought therefore to 
support or refute our hypothesis. It belongs 
to a swimming animal, and should therefore 
have the sinus and aperture and striae of 
growth as in Nautiloidea ; and these it certainly 
has. We can appeal to this example as a 
most convincing exception to prove the rule 
that the shell is a true index of the most re- 
markable adaptive structures, and, among the 
fossils, can give us exact information of im- 
portant differences in structure and habits. 
The efforts of the Orthoceratite to adapt 
itself fully to the requirements of a mixed habi- 
tat gave the world the Nautiloidea: the efforts 
of the same type to become completely a lit- 
toral crawler developed the Ammonoidea. The 
successive forms of the Belemnoidea arose in 
the same way; but here the ground-swimming 
habitat and complete fitness for that was the 
object, whereas the Sepioidea represent the 
highest aims as well as the highest attainments 
of the Orthoceratites, in their surface-swim- 
ming and rapacious forms. 
We cannot seriously imagine these changes 
to have resulted from intelligent effort; but 
we can fully join Lamarck,’ Cope, and Ryder, 
in imagining them as due to efforts induced by 
the physical requirements of the habitat, and 
1 A noted French writer well known to embryologists, La- 
caze-Duthier, has lately asked, ‘*‘ Who, at the present time, sup- 
ports Lamarck?” The author can answer, that some of our 
leading scientific men consider Lamarck’s hypothesis to contain 
more fundamental truths than Darwin’s or any other. 
SCIENCE. 
125 
think this position to be better supported by 
facts than any other hypothesis.? 
Confining ourselves to the Tetrabranchiata, 
which we think the most favorable for our pur- 
poses, the next problem presenting itself is 
whether the two orders, Nautiloidea and Am- 
monoidea, have had a common origin, or wheth- 
er they bear internal evidence of having had a 
distinct origin. The embryo of all Ammonoi- 
dea, as shown by the author in his ‘ Embryol- 
ogy of the fossil cephalopods of the Museum 
of comparative zodlogy,’ and since confirmed 
by the more extensive researches of Dr. Bran- 
co, is the little bag-like shell first discovered 
by Saemann. This is attached to the apex of 
the secondary shell. ‘The embryonic bag is 
called the protoconch by Professor Owen ; and 
the secondary or true shell, the conch. 
There is no protoconch in Nautiloidea, as 
first shown by Saemann, then by Barrande, 
and subsequently by the author and Branco ; 
but where it ought to have been attached on 
the apex of the conch, or true secondary shell, 
there is a scar, first demonstrated by Barrande. 
The view brought forward by the author, that 
this sear indicated the former existence of a pro- 
toconch in the Nautiloidea, has been opposed by 
Barrande, Branco, and several authors, on the 
ground that the cicatrix demonstrated the ex- 
istence of a distinct embryonic form. There- 
fore, according to Barrande, the Nautiloidea 
were not similar to the Ammonoidea in their 
earliest stages of growth, and must have been 
equally distinct in origin. 
Our present contribution to this discussion 
is simple and straightforward. We have found 
the protoconch in several forms of Orthocera- 
tites, of some of which we give figures; and, 
further, it can probably be found on the apex 
of the so-called perfect shells, which have no 
sear or cicatrix. ‘These were discovered by 
Dekoninck, and supposed by him, in his ‘ Cal- 
caire carbonifére’ (Ann. du mus. roy. de Bel- 
gique), to be fatal to our conclusion. Having 
no scar, they could not possibly, according to 
DeKoninck, have had a protoconch. When 
the so-called perfect apex is broken off, the 
observer will probably find that this apex was 
the shrivelled remains of a protoconch which 
concealed the cicatrix underneath, as in Fig. 2. 
There is therefore no essential difference be- 
tween the embryos of the Ammonoidea and 
those of the Nautiloidea. ‘There are some of 
1 We can also confidently appeal to Dohrn’s hypothesis of 
change of function in support of this view, in which he shows 
with many convincing examples that organs have latent func- 
tions which can be developed by any change of habits, and then 
become predominant over the older functions, and by their reac- 
tions occasion an entire change in the structure of the organs 
themselves. 
