35) 
which however was 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 excessive growth, as in 
Nautilus pompilius. 
The Ammonoidea in the earlier forms, the Goniatitine of the Silu- 
rian,* had apertures with well-marked ambulatory sinuses sufficient to 
show that they must have had considerable powers of rising or leap- 
ing in the water, if not swimming, like the Nautilus. In the later 
forms of the same suborder and in the Ceratitine, Ammonitine and 
Lytoceratinz the ambulatory sinus is absent; and in its place a 
projecting crest or rostrum was developed indicating reduction in 
size and disuse of the hyponome. ‘This and the generally open 
apertures enable us to see that they were more exclusively bottom- 
crawlers than the Nautiloidea. 
The most interesting of the facts in this order lies among the 
exceptional shells, some of which must have been sedentary, and 
could 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 aper- 
ture which indicated transition to an exclusively swimming habit. 
These shells appear in our subsequent remarks among phylogerontic 
and pathologic types. 
The Belemnoidea of the Jura had a solid cylindrical body, called 
the guard, attached to the cone-like internal shell, and partly 
enclosing it. Aulacoceras of the Trias, as described by Branco, is 
a transitional form with an imperfect guard, which frequently con- 
tains fragments of other shells and foreign matter. This demon- 
strates that this guard could only have been built by some external 
flap or inclosing sac, independent of the true mantle. This false 
mantle must have inclosed both the shell and the guard, and must 
have been at the same time open, so as to admit the foreign mate- 
rials which Branco found built into the substance of the guard. 
One of the straight shells of the Silurian Nautiloidea, Orthocera- 
tutes truncatus, regularly breaks off the cone of its shell, and then 
mends the mutilated apex with a plug. This plug, we are able to 
* See Plate ii. 
