496 MESOZOIC TIME REPTILIAN AGE. 



nized, 70 Jurassic, and less than a dozen Cretaceous. Only a small part — pro- 

 bably not a tenth — of tlie species that actually existed would have become fos- 

 silized, as they are not confined to swampy soil. 



Culmination of the Cephalopod type, and therefore of the type of Mol- 

 hisks. — The Cephalopods are the highest of Mollusks, and in their 

 culmination the culmination of the grand type of Mollusks took 

 place. 



In the Triassic period, before the disappearance of the Gonia- 

 tites, — a Palaeozoic form of the Ammonite type, — true Ammonites 

 appeared, and the family rapidly multiplied in species and variety 

 of forms. Between 800 and 900 species have been found fossil in 

 the Mesozoic rocks. Besides these, the Belemnite family — charac- 

 terized by an internal shell — commenced in the epoch of the Lias, 

 and over 120 of its species have been gathered from the Jurassic 

 and Cretaceous strata. There were also many species of Nautilus ; 

 so that the whole number of Cephalopods now known from the 

 formations of the era is nearly 1200. Of this number about 950 

 were chambered shells of the Nautilus and Ammonite groups ; 

 while in existing seas there are only four species, and these belong 

 to the single genus of Nautilus. The Ammonite and Belemnite 

 families have had no living representatives since the Cretaceous 

 period. It is to be noted that 950 is the number of species of 

 chambered shells found fossil: it may be only a third part (or 

 less) of those which were actually in the waters of the era. The 

 age was therefore remarkable for the great expansion of the type 

 of Cephalopods. 



The number of species of the Ammonite family from the Triassic is about 100, 

 from the Jurassic 400 (over 160 from the Liassic), from the Cretaceous about 

 400. The genus Nautilus has over 40 Mesozoic species. 



In the families of chambered shells the expansion of the Tetrabranch Cepha- 

 lopods (p. 156) is exhibited; and in the Belemnite, Sepia, and Calamary fami- 

 lies, that of the Dibranchs. 



The Conulariee appear to have been the only representatives of the latter in 

 the Palaeozoic. They are generally referred to the group of Pteropods ; but 

 the large, thin, pyramidal shells, chambered at bottom, as Hall has observed, 

 and admitting of some motion at the angles above, correspond better with the 

 internal shell or osselet of a Cephalopod. 



The type began in the straight Orthoceras with plain septa, and 

 the half-coiled and equally simple Lituite of the Lower Silurian ; 

 it reached its maximum in the large and complex Ammonite, of the 

 Jurassic, and the associated Belemnite and Cuttle-fishes ; it declined 

 in the Cretaceous, through the multiplication of the half-coiled 

 forms of the Ammonite family (p. 485) and the straight Baculite. 



