16 EOCENE MOLLUSC A. 



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 Ammonitida, and connecting that family with the Nautilida, also occur. 

 As we ascend in the Palaeozoic 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 

 Nautilida, which lived during the Palaeozoic 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.f In the Oolitic group, twenty-five species of Belemnite, 

 and remains of various genera belonging to the families Loligida and Teuthida, 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 Ainmonitida 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 Nautili 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 clem rotlien Marmor von Aussee), describes several Ortho- 

 ceratitcs associated with Goniatites in the schistose beds of St. Cassian ; those beds, I believe, are now 

 generally considered to belong to the Muschelkalk. 



f The remains described by Goldfuss and Bronn as Spirulce, appear to belong to Gyroceras, a genus of 

 the Nautilida. 



