212, HISTORICAL PALZONTOLOGY. 
characterised by the occurrence in them of the Scaliop, Pecten 
Valoniensts (fig. 144, 6); the small Cockle, Cardium Rheticum 
(fig. 144, 2); 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 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 Zetrabranchiates, 
but also, for the first time, by the internal skeletons of Dzbran- 
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 Belemnitie ; 
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 Pale- 
ozoic Orthoceratites, some of them of considerable size, along 
with the ancient Cyrtoceras and Gontatites ; 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 Asmonites, 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- 
niatitesand Ammonites 
in the fact that the 
Fig. 145 —Ceratites nodosus, viewed from the side ares 
a and from behind. Muschelkalk. septa or partitions be- 
tween the air-cham- 
bers are not simple and plain (as in the /Vaw/7/us and its allies), 
but are folded and bent as they approach the outer wall of the 
shell. In the Gonzatite these foldings of the septa are of a simply 
lobed or angulated nature, and in the Ammonite they are ex- 
