ORTHOCERATITE. 363 
Orthoceratite, PL 44, Fig. 4. 
The Orthoceratites (so called from their usual 
form, — that of a straight horn) began their exist- 
ence at the same early period with the Nautili, 
in the seas which deposited the Transition strata; 
and are so nearly allied to them in structure, 
that we may conclude they performed a similar 
function as floats of Cephalopodous MoUusks. 
This genus contains many species, which abound 
in the strata of the Transition series, and is one 
of those which, having been called into existence 
amongst the earliest inhabitants of our planet, 
was at an early period also consigned to almost 
total destruction.* 
An Orthoceratite (see PI. 44, Fig. 4) is, like 
the Nautilus, a multilocular shell, having its 
chambers separated by transverse plates, concave 
externally, and internally convex ; and pierced, 
either at the centre or towards the margin, by a 
Siphuncle, (a.) This pipe varies in size, more 
* See D'Orbigny's Tableau Methodique des Cephalopodes. 
There are, I believe, only two exceptions yet known to the 
general fact, that the genus Orthoceratite became extinct before 
the deposition of the Secondary strata had commenced. The 
most recent rocks in which they have been noticed, are a small 
and problematical species in the Lias at Lyme, and another 
species in Alpine Limestone of the Oolite formation, at Halstadt, 
in the Tyrol. 
