124 
dicated transition to an exclusively swimming 
habit. These shells appear in our subsequent 
remarks among the geratologous and pathologi- 
cal types. 
The shells of this order have no such vari- 
ety of form in the paleozoic formations as we 
have described in the Nautiloidea. They are 
close coiled, and even involute, in some of the 
first forms found in the Cambrian. 
The Belemnoidea of the Jura had a solid 
cylindrical body, called the guard, attached 
to the cone-like internal shell, and partly en- 
closing it. Aulacoceras of the trias, as de- 
scribed by Branco, is a transitional form with 
an imperfect guard, which frequently contains 
fragments of other shells and foreign matter. 
This demonstrates an important link in our 
evidence, that this guard could only have been 
built by some external flap or enclosing sac, 
independent of the true mantle. This false 
mantle must have enclosed both the shell and 
the guard, and must have been at the same 
time open, so as to admit the foreign materials 
which Branco found built into the substance of 
the guard. One of the straight shells of the 
Silurian Nautiloidea, Orthoceratites truncatus, 
regularly breaks off the cone of its shell, and 
then mends the mutilated apex with a plug. 
This plug, we are able to say, is the precise 
homologue, in position and in structure, of the 
guard of the Belemnite. Barrande showed 
this plug to have been secreted by external 
organs, as he supposed, — two arms stretching 
back from the aperture like those of Argo- 
nauta, and reaching beyond the broken apex. 
The dorsal fold of Nautilus is, however, a secret- 
ing-organ stretching back over the shell; and, 
as the probable homologue of the plug-secret- 
ing organ of the Orthoceratites and the guard- 
building organ of the Belemnoidea, it enables 
us at once to explain how the Belemnoidea 
arose from the Orthoceratites, and why Aula- 
coceras had an imperfect mantle. ‘This fold, 
which was far larger among the ancient Ortho- 
ceratites, would have been necessarily open on 
the ventral side, then more but not completely 
closed in Aulacoceras, and finally completely 
closed in the later Belemnoidea, and able to 
construct a guard as perfect as that which 
they carry. 
The solid guard of sissies animals in a com- 
pact cylindrical body, such as they were known 
to possess, could have been only a heavy bur- 
den to a swimming animal. The Belemnoidea, 
therefore, were not purely natatory; but for 
these and other reasons, which we cannot here 
discuss, they were evidently ground-swimmers, 
probably boring into the mud for shelter, or as 
SCIENCE. 
a means of concealing themselves while lying” 
in wait for their prey. 
The old view, that the guard could have been 
in any sense a ‘ guard’ against collisions with 
rocks, etc., in their wild leaps backwards, is 
inadmissible for many reasons. ‘The most ob- 
vious are its position as an internal organ, its 
solid structure, and its weight. We think it 
more reasonable to suppose that it might have 
increased the liability to injury from collisions. 
In tracing the Belemnoidea to the Orthocera- 
tites we have simply continued the labors, and 
carried out more fully the sagacious inferences, 
of Quenstedt and Von Ihering. 
The modern Sepioidea are known to be 
almost exclusively swimming types; and the — 
more ancient, normal, flattened forms, and their 
descendants the existing cuttlefishes, have 
flattened internal shells, in which the striae of 
srowth are remarkable for their forward inflec- 
tion on the dorsal aspect, due to the immense 
comparative length of this side of the aperture. 
Gonioceras, a well-known Silurian type of the 
orthoceratitic Nautiloidea, has the same con- 
tours in the striae of growth on the dorsum ; 
and if, as we think, it had a corresponding de- 
pression in the aperture on the ventral side, 
in similar proportion to that of other forms, 
the aperture must have been transitional to the 
internal shell of Paleoteuthis Dunensis of the 
Devonian, and to the more modern forms. 
The septa, also, of Gonioceras, have similar 
curves to the layers of calcareous matter in 
the interior of the cuttlefish bone, which we 
look upon as aborted and retrogade homo- 
logues of the septa of other forms. Gonio- 
ceras connects directly with a series of less 
compressed, straight, orthoceratitic shells ; and 
thus the independent derivation of the Sepioi- 
dea from the Orthoceratites, among the shell- 
covered, coniform Tetrabranchiata, is probable. 
The enclosure and suppression of the shell have 
already been predicted, with a sagacity which 
commands our highest admiration, by Lankes- 
ter, from studies of the embryo of Loligo; and 
these facts carry out his conclusions, substi- 
tuting, however, the more ancient Sepioidea for 
the Belemnoidea, with which Lankester made 
his comparisons, and the hood for the two 
mantle-flaps which were imaged by him as the 
organs which enclosed the shell and formed 
the shell-sac. Most paleontologists have con- 
sidered the Sepioidea and Belemnoidea as more 
closely allied ; but they appear to us as two 
orders, certainly as distinct as, and perhaps 
even more widely divergent than, the Nauti- 
loidea and Ammonoidea. 
Among these two orders we recognize many 
[Vou. III., No. 52. 
