m 



In this class of Molluscs we find the highest invertebrate organisation ; 

 in fact when we consider the distinctness of functions assigned to their 

 different organs, the acuteness of their powers of siglit, and, in most of the 

 existing species, of hearing, the size of their cephalic ganglia, which may 

 almost be regarded as a brain, and the possession of a rudimentary interna 

 skeleton protecting these organs, we cannot help assigning them a place in the 

 animal kingdom above the gelatinous fishes. They derive theii' name from the 

 locomotive and prehensile organs that radiate from the head ; they are aquatic 

 and breathe by gills (hranchiw), concealed beneath a mantle which has two 

 openings, one like a slit for the entrance of water, the other funnel-shaped for 

 its exit ; they propel themselves by the forcible expulsion of water from the 

 respiratory chamber. 



The class is divided into two orders, according to the number of gills. 



Ko remains of the Dibranchiata or two-gilled Cephalopods have been 

 found before the phragmocones of Belemnitfs in the Lower Lias. Lyell (Principles, 

 p. 152, ed. 1867) calls attention to the fact that several of the existing species are 

 soft-bodied, and that such might have lived in t)i« Palaeozoic seas, but have 

 left behind them no memorials of their existence. Tliey progressed in 

 number duiing the deposition of the Secondary formations, and are now 

 abxmdantly represented in the poulps, squids, cuttle-fishes, &c., of every sea. 



The Silurian Cephalopods belong entirely to the Tetrabranchcata or 

 four-gilled order, and to this also are referred the Ammonitida:, which, beginning 

 with the upper Devonian Goniatites, chaxacterise afterwards in such profusion 

 the secondary rocks. 



McCoy and Sedg\vick (Palaeozoic Fossils, Camb. 1855), class the Orthoceraa 

 and allied genera with the Nautilus and Lituites in one family Nautilidce. 

 Woodward (Recent and Fossil Shells) gives a family Orthoceratidat, keeping with 

 the Nautilidce the genera Lituites and Trochoceras. Owen (Palaeontology, p. 99, 

 •d. 1861), suggests the possible removal of all the paheozoic so-called Nautilidce 

 into the family Orthoceratidce. "WTiatever division we adopt, we shall get the 

 best idea of an Orthoceras by imagining the recent pearly nautilus (Nautilus 

 pompilius) unroUed, and of a Lituites the same with the apex spiraL Good 

 sections of this mollusc, which should be studied by all who wish to understand 

 the physiology of the Silurian Cephalopoda, are given in Owen's Palaeontology 

 and in the frontispiece of Woodward's Recent and Fossil Shells. 



The tetrabranchiate Cephalopod has left as its fossil memorial a shell 

 sometimes straight sometimes curved, its transverse section being either circular 

 or elliptical. This shell is divided into chambers, best seen in the Fossil 

 Sketches, Plate ii. ; the several chambers being parted from one another by 

 layers of shelly matter called septa. These are generally concave on the face 

 which looks towai'd the outer chamber, and show themselves on the surface by 

 lines or rings, most simple in the Nautilidce and Orthoceratidce, but in the 



