16 EOCENE MOLLUSCA. 
useless to them. We may assume, therefore, that in the extinct Cephalopods the 
presence of the mucro or rostrum will indicate a littoral animal. M. d’Orbigny states 
that he has always observed in the Sepia, the extremity of the mucro projecting 
beyond the body, and it is not improbable that this part of the shell may be used, as 
he suggests, for defensive purposes, and that it is protruded at the will of the animal. 
The Cephalopods, highly organized as they are in comparison with the other 
molluscs, are among the earliest forms of animal life which geology has brought to light. 
The Silurian group, the most ancient fossiliferous formations with which we are 
acquainted, contains the remains of one species of Nautilus, and of many species be- 
longing to cognate genera. Several species of Goniatites, an anomalous genus be- 
longing to the Ammonitide, and connecting that family with the Wawtilide, also occur. 
As we ascend in the Palzeozoic series, we find that various of the primitive genera and 
species disappear, and are succeeded by other forms, distinct from, although closely 
allied to, them ; which, in their turn, are also lost. On passing into the Mesozoic 
series a marked change takes place. Of the eight genera constituting the family 
Nautilide, which lived during the Paleozoic epoch, Orthoceras* and Nautilus alone 
survive ; and of the long series of species belonging to the latter genus, whose remains 
are found in the carboniferous formations, every one disappears ; but an immense array 
of Ammonites starts into existence, with septa at first comparatively simple, but be- 
coming more complicated in structure in the succeeding formations. The dibranchiate 
Cephalopods now first appear.t In the Oolitic group, twenty-five species of Belemnite, 
and remains of various genera belonging to the families Loligide and Teuthide, have 
been found. The Belemnites occur in incredible quantities, and sometimes form entire 
strata. Passing into the Cretaceous group, we still find the Nautilus, though of 
diminished importance ; the Ammonites are reduced in number to little more than a 
fourth part of the species found in the Oolitic group, and new modes of convolution appear 
in their shells, on which the several other genera constituting the family 4mmonitide are 
founded. The family itself gradually diminishes as we ascend in the Cretaceous group, 
and wholly disappears with the secondary period. The Belemnites appear to be the 
sole representatives of the dibranchiate Cephalopods during this epoch, and with it 
they also perish. On entering into the tertiary formations we find, that of the rich 
and varied assemblage of tetrabranchiate Cephalopods which characterised the fauna 
of the secondary period, only the Wawfi/i survived. On the Continent their remains 
are found in the Eocene formations, and also in the Miocene formations, at Turin and 
in Touraine ; but in this country they are confined to the older Eocene deposits. Of 
* Von Hauer (Nene Cephalopoden aus dem rothen Marmor yon Aussee), describes several Ortho- 
ceratites associated with Goniatites in the schistose beds of St. Cassian; those beds, I believe, are now 
generally considered to belong to the Muschelkalk. 
+ The remains described by Goldfuss and Bronn as Spirule, appear to belong to Gyroceras, a genus of 
the Nautilide. 
