. The ancient life-history of the earth; a comprehensive outline of the principles and leading facts of palæontological science. Paleontology. 212 HISTORICAL PALEONTOLOGY characterised by the occurrence in them of the Scallop, Pecten Valonietisis (fig. 144, b)\ the small Cockle, Cardiuni Rhceticum (fig. 144, d)\ and the curiously-twisted Pearl-oyster, Avicula contorta (fig. 144, e)—this last Bivalve being so abundant that the strata in question are often spoken of as the " Avicula contorta ; Passing over the groups of the Heteropods and Pteropods^ we have to notice the Cephalopo
. The ancient life-history of the earth; a comprehensive outline of the principles and leading facts of palæontological science. Paleontology. 212 HISTORICAL PALEONTOLOGY characterised by the occurrence in them of the Scallop, Pecten Valonietisis (fig. 144, b)\ the small Cockle, Cardiuni Rhceticum (fig. 144, d)\ and the curiously-twisted Pearl-oyster, Avicula contorta (fig. 144, e)—this last Bivalve being so abundant that the strata in question are often spoken of as the " Avicula contorta ; 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 Tetrabraiichiates, but also, for the first time, by the internal skeletons oi Dibraji- chiate forms. The Trias, therefore, marks the first recognised appearance of true Cutde-fishes. All the known examples of these belong to the great Mesozoic group of the Bcleuuiitidce; 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 Ort/wceratites, 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 seas, and which will be spoken of here- after. The form, how- ever, which is most characteristic of the Trias is Ceratites (fig. 145). In this genus the shell is curved into a flat spiral, the volu- tions of which are in contact; and it further agrees with both Go- niatites and Ammonites in the fact that the. Fig. 145 —Ceratites nodosus, viewed
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Keywords: ., bookaut, bookcentury1800, bookdecade1870, booksubjectpaleontology
