DEVONIAN AND OLD RED PERIOD. 



149 



(fig. 100). The latter, with its fragile, conical, and often beauti- 

 fully ornamented shell, is especially noticeable. 



The remains of Cephalopoda are far from uncommon in the 

 Devonian deposits, all the known forms 

 being still Tetrabranchiate. Besides the 

 ancient types Oj'thoceras and Cyrtoceras, 

 we have now a predominance of the 

 spirally-coiled chambered shells of Goni- 

 atites and Clyvienia. In the former of 

 these the shell is shaped like that of the 

 Nautilus ; but the partitions between the 

 chambers ("septa") are more or less 

 lobed, folded, or angulated, and the 

 *' siphuncle" runs along the back or con- 

 vex side of the shell — these being char- 

 acters which approximate Goniatites to 

 the true Ammonites of the later rocks. 

 In Clymenia, on the other hand, whilst 

 the shell (fig. 10 1) is coiled into a flat 

 spiral, and the partitions or septa are 

 simple or only slightly lobed, there is still 

 this difference, as compared with the Nautilus^ that the tube of 

 the siphuncle is placed on the inner or concave side of the 



Fig. 100.— Coniiiaria or- 

 nata, of the natural size. 

 Devonian, Europe. 



Fig. loi. — Clymenia Sedgwickii. Devonian, Europe. 



shell. The species of Clymenia are exclusively Devonian in 



