MESOZOIC TIME. 483 



dina, Melania (?), Nerita, Pterocera, Tellina, Corbis, Anomia, etc. Tn the Cretaceous: 

 Neithea, Crassatetta, Aodncea (Pectunculus), Petricola, Venus (f ), Oliva, Ovula, Cyprcea, 

 Voluta, Turns {Pleurotoma), Pseudobuccinum, etc. 



Cephalopods, the highest of Mollusks, culminated in the Mesozoic ; 

 and, in their culmination, the culmination of the grand type of Mol- 

 lusks took place. This fact is strikingly exhibited in the history of 

 the Ammonite and Belemnite groups. The genus Goniatites, a Pal- 

 eozoic form of the Ammonite type, ended in the Triassic ; but before 

 this the earliest Ammonites had already appeared ; and these continued 

 afterward to increase in variety and numbers through the Mesozoic. 

 Nearly 1,000 species of the Ammonite family 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. In 

 existing seas, there are only four species of chambered external shells ; 

 and these belong to this latter genus. The Ammonite and Belemnite 

 families died out, or nearly so, with the close of the Cretaceous period. 

 It is to be noted that the above are the numbers of species of cham- 

 bered shells found fossil: it may be but a small part of those which 

 "were actually in the waters of the era. The age was therefore re- 

 markable for the great expansion of the type of Cephalopods. 



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

 the half-coiled and equally simple Lituites of the Lower Silurian ; it 

 reached its maximum in the large and complex Ammonite of the Ju- 

 rassic, and the associated Belemnite and Cuttle-fishes ; it declined in 

 the later Mesozoic, through the multiplication of the half-coiled forms 

 of the Ammonite family (p. 462) and the straight Baculite ; and, at 

 the close of the period, there was a sudden disappearance of genera 

 and species. Whether any of the modern Cuttle-fishes (Dibranchs) 

 are equal, or superior, to the highest Cephalopods of the Jurassic, it 

 is difficult to determine. The modern germs Nautilus — representing 

 the chambered species (Tetrabranchs) — is certainly of far lower 

 grade than the Jurassic Ammonite. 



It is therefore one of the great facts connected with the Mesozoic 

 era that, in its later half, the sub-kingdom of Mollusks passed its period 

 of culmination. But, while this is true of the sub-kingdom as a 

 whole, it is not true of each of its subdivisions ; for the inferior tribes 

 of Lamellibranchs and Gasteropoda continue on the rising grade 

 through the Mesozoic, and probably have their maximum display at 

 the present time. 



Articulates. — The class of Crustaceans rose to Macrurans 

 (Shrimps and Lobsters) and true Crabs ; and among the latter all the 

 higher divisions were represented. The class of Insects was also un- 



