CEPHALOPODA. 1838 
nautilus (N. arietis, Reinecke, —N. bidorsatus, Schlotheim), is 
found, and two kinds of rhyncholite; one sort, corresponding 
with the upper mandible of the recent nautilus, has been called 
‘“‘rhyncholites hirundo” (Pl. II., Fig. 11); the other, which 
appears to be only the lower mandible of the same species, has 
been described under the name of ‘‘conchorhynchus avirostris.”’* 
They also occur in the belemnite beds of the middle las of 
Dorsetshire ; these latter are very different in form from those 
of nautili in the lower lias, and may probably belong to 
belemnites. 
In studying the fossil tetrabranchiata, it is necessary to take 
into consideration the varying circumstances under which they 
have been preserved. In some strata (as the lias of Watchett) 
the outer layer of the shell has disappeared, whilst the inner 
nacreous layer is preserved. More frequently only the outer 
layer remains; and in the chalk formation the whole shell has 
perished. In the calcareous grit of Berkshire and Wiltshire the 
ammonites haye lost their shells; but perfect casts of the 
chambers, formed of calcareous spar, remain.+ 
Fossil orthocerata and ammonites are eyidently in many 
instances dead shells, being overgrown with corals, serpulz, or 
oysters ; every cabinet affords such examples. In others the 
animal has apparently occupied its shell, and prevented the 
ingress of mud, which has hardened all around it; after this it 
has decomposed, and contributed to form those phosphates and 
sulphides commonly present in the body-chamber of fossil shells, 
and by which the sediment around them is so often formed into 
a hard concretion.{ In this state they are permeated by mineral 
water, which slowly deposits calcarecus spar, in crystals, on 
their walls; or by acidulous water, which remoyes every trace 
of the shell, leaving a cavity, which at some future time may 
again become filled with spar, having the form of the shell but 
not its structure. In some sections of orthocerata it is evident 
that the mud has gained access to the air-cells; but the cham- 
bers are not entirely filled, because their lining membrane has 
contracted, leaving a space between itself and certain portions 
of the walls, which correspond in each chamber. 
The tetrabranchs could undoubtedly swim, by their respira- 
tory jets; but the discoidal nautili and ammonites are not well 
* Lepas avirostris (Schlotheim), described by Blainville as the beak of a brachiopod} 
f Called spondylolites by old writers. 
t In he alum-shale of Whitby innumerable concretions are found, which, when 
struck with the hammer, split open and disclose an ammonite. See Dr. Mantell’s 
“Thoughts on a Pebble,” p. 21. , 
