276 MOLLUSCA. 



Fam. i. Nautilidtz. 



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

 or near the concavity of the curved shells, simple. 

 Sub-family Nautilidce proper. 



Body-chamber capacious ; aperture simple ; siphuncle 

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

 cei'as. 

 Sub-family Orthoceratidfe. 



Shell straight, curved, or discoidal ; body-chamber 

 small ; aperture contracted ; siphuncle complicated. 111. 

 Gen. Orthoceras, Phragmoceras, Cyrtoceras. 

 Fam. 2. Amnwnitidce. 



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

 elongated ; aperture guarded by processes, or closed by an oper- 

 culum ; sutures angulated, lobed, or foliaceous ; siphuncle external 

 or dorsal (on the convex side of the curved shells). 111. Gen. 

 Ammonites, Ceratites, B acuities, Turrilites, Scaphites, Ancyloceras. 



As regards their general distribution in time, the Cephalo- 

 pods are largely represented in all the primary groups of strati- 

 fied rocks from the Lower Silurian up to the present day. Of 

 the two orders of Cephalopoda, the Tetrabranchiata is the oldest, 

 attaining its maximum in the Palaeozoic period, decreasing in 

 the Mesozoic and Kainozoic epochs, and being represented at 

 the present day by the single form, Nautilus pompilius. Of the 

 sections of this order, the Nautilidce proper and the Orthocer- 

 atidce are pre-eminently Palaeozoic, and the Ammonitida are 

 not only pre-eminently but are almost exclusively Secondary. 

 Of the abundance of the two former families in the Silurian 

 seas some idea may be obtained when it is mentioned that over 

 a thousand species have been described by M. Barrande from 

 the Silurian basin of Bohemia alone. ThzNautitida proper have 

 gradually decreased in numbers from the Palaeozoic, through 

 the Secondary and Tertiary periods, to the present day. The 

 Orthoceratidcz died out much sooner, being exclusively Palaeo- 

 zoic, with the exception of the genera Orthoceras itself and 

 Cyrtoceras, which survived into the commencement of the Se- 

 condary period, finally dying out in the Trias. 



The second family of the Tetrabranchiata viz., the Ammo- 

 nitidce. is almost exclusively Secondary, being very largely re- 

 presented by numerous species of the genera Ammonites, Cera- 

 lites, Baculites, Turrilites, &c. The only Palaeozoic genera are 

 Goniatites and Bactrites, of which the former is found from the 

 Upper Silurian to the Trias, whilst the latter is a Silurian 

 and Devonian form. The genus Ceratites is characteristically 

 Triassic, but it is said to occur in the Devonian Rocks, and 

 some species are Cretaceous. All the remaining genera are 

 exclusively Secondary, the genera Baculites, Turrilites, Hamites 

 and Ptychoceras being confined to the Cretaceous period. 



