212 HISTORICAL PALAEONTOLOGY. 



characterised by the occurrence in them of the Scallop, Pecten 

 Valoniensis (fig. 144, b}; the small Cockle, Cardium Rhceticum 

 (fig. 144, d) and the curiously-twisted Pearl-oyster, Avicula 

 contorta (fig. 144, <?) this last Bivalve being so abundant that 

 the strata in question are often spoken of as the " Avicula 

 contorta beds." 



Passing over the groups of the Heteropods and Pteropods, 

 we have to notice the Cephalopoda, which are represented in 

 the Trias not only by the chambered shells of Tetrabranchiates, 

 but also, for the first time, by the internal skeletons of Dibran- 

 chiate forms. The Trias, therefore, marks the first recognised 

 appearance of true Cuttle-fishes. All the known examples of 

 these belong to the great Mesozoic group of the BelemnitidcB ; 

 and as this family is much more largely developed in the suc- 

 ceeding Jurassic period, the consideration of its characters 

 will be deferred till that formation is treated of. Amongst the 

 chambered Cephalopods we find quite a number of the Palae- 

 ozoic Orthoceratites, some of them of considerable size, along 

 with the ancient Cyrtoceras and Goniatites ; and these old types, 

 singularly enough, occur in the higher portion of the Trias 

 (St Cassian beds), but have, for some unexplained reason, not 

 yet been recognised in the lower and equally fossiliferous 

 formation of the Muschelkalk. Along with these we meet for 

 the first time with true Ammonites, which fill such an extensive 



place in the Jurassic 



y -^_ seas, and which will 



f " ^V> be spoken of here- 



""^ after. The form, how- 

 rf ever, which is most 

 characteristic of the 

 Lij^ Trias is Ceratites (fig. 

 145). In this genus 

 vj\ the shell is curved into 

 . (^ a flat spiral, the volu- 

 tions of which are in 

 ^ contact ; and it further 

 HsJ. agrees with both Go- 

 ~M niatites and Ammonites 

 in the fact that the 

 eside septa or partitions be- 

 tween the air-cham- 

 bers are not simple and plain (as in the Nautilus and its allies), 

 but are folded and bent as they approach the outer wall of the 

 shell. In the Goniatite these foldings of the septa are of a simply 

 lobed or angulated nature, and in the Ammonite they are ex- 



