FEBRUARY 1, 1884.| 
correlations of habit and structure to the eluci- 
dation of ordinal differences. 
The class Cephalopoda is composed of exclu- 
sively aquatic and marine animals, and conse- 
quently they breathe with gills. The structures 
of the two sub-classes coincide with two dis- 
tinct habitats which they respectively occupy. 
The Tetrabranchiata, like the Nautilus, were 
essentially littoral crawlers, though possessing 
organs suitable for swimming, and doubtless 
using them more or less for leaping and swim- 
ming. 
The animal of the Nautilus has a large man- 
tle or fleshy sac enclosing the internal organs, 
which can be opened around the margin, or 
closed, at the will of the animal. Admitting 
the water around the margin, they fill their 
mantle-cavities with water, and then, closing 
and compressing the mantle-sac, force it out 
with violence through a fleshy pipe, which is 
exclusively used for that purpose, and always 
situated on the ventral side. ‘The reaction of 
the stream is sufficiently powerful to drive the 
body of the animal with varying degrees of 
swiftness backwards. The fleshy pipe is 
therefore an ambulatory pipe or hyponome ; 
and we propose, in place of the old and con- 
fusing terms, to call it by this name. 
The Dibranchiata change the external shell, 
which they inherit from the Tetrabranchiata, 
into an internal organ, and taking -advantage 
of the powerful hydraulic apparatus of the 
Tetrabranchiata, which they also inherit, and 
increasing its efficiency, become, as is well 
known, exclusively swimmers. 
The ambulatory pipe of the Nautilus causes 
a corresponding depression or sinus to occur 
in the aperture of the shell on the outer or 
ventral side, and its effect is also to be seen 
in the striae of growth throughout the entire 
length of the shell on the ventral side; so that 
we know, from these indications in any fossil, 
what was the comparative size of the pipe, and 
whether the animal was more or less power- 
ful as a swimmer. Other indications, such as 
the openness or contracted form of the vari- 
ous apertures of different genera, exhibit with 
equal clearness what they could do in the way 
of crawling. ‘The wide-open apertures indi- 
cate powerful arms, capable of carrying and 
easily balancing the large spire of the shell 
above: the narrow contracted aperture shows 
that the arms were small, and that the animal 
could not so efficiently balance or carry the 
shell in an upright position, and was there- 
fore, according to the amount and style of the 
contraction, more or less inefficient as a 
crawler. 
SCIENCE. 
123 
In studying the different types of the Tetra- 
branchiata, we find that there are two orders 
as first defined by Professor Louis Agassiz, — 
the Nautiloidea and the Ammonoidea, — and, 
further, that these divisions coincide with dif- 
ferences in the outlines of the ambulatory 
sinuses which indicate distinctions of habit 
general throughout each order. 
The extinct Nautiloidea have large ambula- 
tory sinuses, and were evidently capable, like 
the modern Nautilus, of rising to the surface, 
and swimming with a jerky motion; though 
their open apertures, as a rule, show their nor- 
mal condition to have been crepitant, or bot- 
tom-crawling. The exceptional shells, which 
depart from the typical form in the sinus and 
apertures, exhibit their peculiarities in the 
adults, but not, as a rule, in the young, except 
in cases where direct inheritance can be proven 
to have occasioned the exception. ‘The excep-— 
tions, then, are, in fact, the most conclusive of 
our proofs, since they show the power of the 
habitat to produce permanent changes in the 
apertures. 
The orthoceratitic shells of this order are 
straight cones, with internal septa dividing 
them into air-chambers, connected by a tube 
uniting all the air-chambers, and opening into 
the body of the animal itself, which occupied 
a small part only of the whole length of the 
cone. ‘This is the simplest form: and others 
are, the bent or arcuate, cyrtoceratitic; the 
loosely coiled, but with whorls not in contact, 
gyroceratitic ; the closely coiled, with whorls 
in contact, nautilian ; and the still more closely 
coiled or involute shells, the involute nautilian, 
in which the outer whorls may simply overlap 
the inner, or entirely conceal them by their ex- 
cessive growth, as in Nautilus pompilius. 
The Ammonoidea in their earlier forms, the 
Goniatites, have apertures, with a less strongly 
marked ambulatory sinus, but still sufficient 
to show that they must have had considerable 
powers of rising or leaping in the water, if not 
of swimming, like the Nautilus. In their later 
forms, the Ammonitinae, however, the ambula- 
tory sinus is absent; and in its place project- 
ing beaks or rostra are developed, indicating 
reduction in the size and use of the ambula- 
tory pipe. ‘This and the generally open aper- 
tures enable us to see that they were more 
exclusively bottom-crawlers than the Nautiloi- 
dea. ‘The most interesting of the facts in this 
order lies among the exceptional shells, some 
of which must have been sedentary, and neither 
have crawled nor moved about with any ease ; 
but none of these, so far as we know, seems 
to have exhibited a type of aperture which in- 
