366 BACULITE. 
Orthoceratite, so we find in the Cretaceous 
formation alone, the remains of a genus which 
may be considered as a straight Ammonite. 
(See PI. 44, Fig. 5.) 
The Baculite (so called from its resemblance 
to a straight staff) is a conical, elongated, and 
symmetrical shell, depressed laterally, and di- 
vided into numerous chambers by transverse 
plates, which, like those in the Ammonite, are 
sinuous, and terminated by foliated dentations 
at their junction with the external shell ; being 
thus separated into dorsal, ventral, and lateral 
lobes and saddles, analogous to those of Ammo- 
nites.* 
It is curious, that this straight modification 
of the form of Ammonites should not have ap- 
peared, until this Family had arrived at the last 
stage of the Secondary deposits, throughout 
which it had occupied so large an extent ; and 
that, after a comparatively short duration, the 
Baculite should have become extinct, simul- 
taneously with the last of the Ammonites, at the 
termination of the Chalk formation. 
* The external chamber (a) is larger than the rest, and swel- 
ling; and capable of containing a considerable portion of the 
animal. The outer shell was thin, and strengthened, like the 
Ammonite, by oblique ribs. Near the posterior margin of the 
shell, the transverse plates are pierced by a Siphuncle (PI. 44, 
5^, c,). This position of the Siphuncle, and the sinuous form 
and denticulated edges of the transverse plates, are characters 
which the Baculite possesses in common with the Ammonite. 
