NAUTILID^i. 



283 



The genus Trochocems is one which was founded by M. 

 Barrande to include certain singular Silurian Cephalopods in 

 which the shell is doubly curved. In the typical forms 

 corresponding with Turrilites amongst the Ammonitidcz the 

 coils of the shell are in contact and pass obliquely round a 

 central axis, so that the shell becomes turreted. In other 

 cases, however, the shells are simply bent, and we have an 

 approach to the genus Cyrtoceras, 



In the genus Clymenia (fig. 252) the shell is discoidal, coiled 

 into a flat spiral, and closely resembling some of the older 

 forms of Nautilus. The inner side of each whorl is deeply 

 excavated for the reception of the convexity of the internal 

 whorl. The septa are simple, like those of Nautilus, or are 

 slightly lobed, and the siphuncle is internal, placed on the 

 concave side of the whorls. Numerous species of Clymenia are 

 known, all belonging to the Devonian period ; and some of 

 the Upper Devonian limestones of 

 Germany are so profusely charged 

 with fossils of this genus as to have 

 received the name of " Clymenien- 

 kalk." 



SUB - FAMILY 2. ORTHOCERAT- 

 IDTE. Shell straight, curved, or 

 discoidal ; body - chamber small ; 

 aperture of the shell small, some- 

 times extremely contracted; siph- 

 uncle complicated. The Ortho- 

 ceratidcz commence in the lowest 

 Silurian deposits, and attain their 

 maximum of development in the 

 Silurian Rocks. The family is well 

 represented in the Devonian and 

 Carboniferous Rocks, but is much 

 reduced in numbers in the Per- 

 mians. The last appearance of the 

 family is in the Triassic Rocks, 

 where it is represented by the 

 genera Orthoceras and Cyrtoceras. 

 The chief genera of this sub- 

 family are Orthoceras, Gompho- 

 ceras, Phragmoceras, Cyrtoceras, 

 Gyroceras, and Ascoceras. 



In the genus Orthoceras (fig. 253) the shell is straight, the 

 siphuncle central or excentric, often of a very complex struc- 

 ture, and the aperture of the shell sometimes contracted. The 



253. Urtliocen 

 septum. Lower Silurian. The lower 

 figure is a section, showing the form 

 and position of the siphuncle. 



