58 CEPHALOPODA. 



Fam. 5. Sepiada. 



Shell calcareous, consisting of a broad, laminar plate, termi- 

 nating in an imperfectly- chambered aj)ex (" phragmacone "). 

 111. Gen. Sepia, Beloptera, S2nrulirostra. 

 Fam. 6. Spirulidce. 



Shell internal, nacreous, chambered, discoidal ; the whorls 

 separate ; a ventral siphuncle. Gen. Spirula. 



Order II. Tetrabraxchiata. 



Animal witli four gills ; arms more than ten, without 

 suckers ; no ink - bag ; shell external, chambered, and si- 

 phuncled. 

 Fam. 1. Nautilidie. 



Sutures of the shell simple ; the siphimcle central, sub- 

 central, or near the concavity of the curved shells, simple. 

 Sub-family Nautilidce proper. 



Body -chamber capacious; ajierture simple; sij^huncle 

 central or internal. 111. Gen. Nautilus, Lituites, Trochn- 

 ceras. 

 8uh-fa7)iihj Orthoceratidee. 



Shell straight, curved, or discoidal; body -chamber 

 small ; aperture contracted ; siphuncle complicated. 111. 

 Gen. Orthoceras, Phragmoceras, Cyrtoceras. 

 Fam. 2. Ammonitidce. 



Shell discoidal, curved, spiral, or straight; body -chamber 

 elongated ; aperture guarded by processes, or closed by an 

 operculum ; 'sutures angulated, lobed, or foliaceous ; siphuncle 

 external or dorsal (on the convex side of the curved shells). 

 111. Gen. Ammonites, Ceratites, Baculites, Turrilites, Scaphites, 

 Ancyloceras. 



As regards their general distribution in time, tlie Cephalo- 

 pods are largely represented in all the primary groups of 

 stratified rocks from the Lower Silurian up to the present 

 day. Of the two orders of Cepludopoda, the Tetrabranchiata 

 is the oldest, attaining its maximum in the Paheozoic period, 

 decreasing in the Mesozoic and Kainozoic epochs, and being 

 represented at the present day l)y the single genus Ncmtilus. 

 Of the sections of this order, the Nautilidce proper and the 

 Orthoccratidm are pre-eminently Palreozoic, and the Ammon- 

 itidce are not only pre-eminently l)ut are almost exclusively 

 Secondary. Of the abundance of the two former families in 

 the Silurian seas some idea may be obtained when it is men- 

 tioned that over a thousand species have been described by 



