DEVONIAN AND OLD RED PERIOD. 



151 



Fig. 99. Different views of Platyceraa du- 

 mosum, of the natural size. Devonian, Can- 

 ada. (Original.) 



forms of this group are known. The type most abundantly 

 represented, especially in America, is Platyceras (fig. 99), 



comprising thin, wide- 

 mouthed shells, probably 

 most nearly allied to the 

 existing " Bonnet-limpets, " 

 and sometimes attaining 

 very considerable dimen- 

 sions. We may also note 

 the continuance of the 

 genus Euomphalus, with 

 its discoidal spiral shell. 

 Amongst the Heteropods, 

 the survival of Bellerophon 

 is to be recorded ; and in the " Winged-snails, " or Pteropods, we 

 find new forms of the old genera Tentaculites and Conularia 

 (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 Orthoceras and Cyrtoceras, 

 we have now a predominance of the 

 spirally-coiled chambered shells of Goni- 

 atites and Clymenia. In the former of 

 these the shell is shaped like that of the 

 Nautilus; but the partitions between tbe 

 chambers ("septa") a're 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 



,, u 11 /c \ -i j o -Fig. 100. Conularia or- 



the shell (ng. 101) is coiled into a flat nata, of the natural [size. 



spiral, and the partitions or septa are De 1 ^. Europe, 

 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 

 shell. The species of Clymenia are exclusively Devonian in 

 their range; and some of the limestones of this period in 

 Germany are so richly charged with fossils of this genus as to 

 have received the name of " Clymenien-kalk. " 



