218 



HISTORICAL PALEONTOLOGY. 



Fig. 145. Ceratites nodosus, viewed from the side 

 and from behind. Muschelkalk. 



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 Ammon- 

 ites in the fact that the 

 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- 

 tremely complex; whilst in the Ceratite there is an inter- 

 mediate state of things, the special feature of which is, that 

 those foldings which are turned towards the mouth of the 

 shell are merely rounded, whereas those which are turned 

 away from the mouth are characteristically toothed. The 

 genus Ceratites, though principally Triassic, has recently been 

 recognized in strata of Carboniferous age in India. 



From the foregoing it will be gathered that one of the most 

 important points in connection with the Triassic Mollusca is 

 the remarkable intermixture of Palaeozoic and Mesozoic types 

 which they exhibit. It is to be remembered, also, that this 

 intermixture has hitherto been recognized, not in the Middle 

 Triassic limestones of the Muschelkalk, in which as the 

 oldest Triassic beds with marine fossils we should naturally 

 expect to find it, but in the St. Cassian beds, the age of which 

 is considerably later than that of the Muschelkalk. The 

 intermingling of old and new types of Shell-fish in the Upper 

 Trias is well brought out in the annexed table, given by Sir 



