PREHISTORIC ANIMALS 



425 



Cephalopoda; some are straight and conical, some conical 

 but slightly curved ; others exhibit a slight attempt at coiling; 

 while still others are more or less closely coiled, either in 

 conical spirals or flat. Some attained 

 a very great size ; thus, in the genus 

 Orthoceras, having a straight coni- 

 cal shell, a Nautiloid of the Palaeo- 

 zoic, the shell attains in some species 

 a length of two meters and a diame- 

 ter of a third of a meter. The oldest 

 representatives of the dibranchiate 

 Cephalopoda are found in the Belem- 

 nites, which had internal, conical shells 

 with chambers ; they extend from the 

 Triassic to the Cretaceous inclusive 

 and have no living representatives. 

 Other dibranchiate Cephalopoda, how- 

 ever, appear in the Mesozoic, and are 

 in their maximum degree of develop- 

 ment at the present day. 



The Mesozoic vertebrates are of 

 great interest. Remains of the Tele- 

 ostei are found first in the Cretaceous, 

 and some of the same genera found 

 there are living to-day, such as the 

 herring and pike. The group of sala- 

 mander-like Amphibia called the Ste- 

 gocephali, represented by the suborder 

 Labyrinth odonta (Gr. Xafivpivdos, 

 labyrinth, and oBovs, tooth), so called 

 because of the complicated structure 

 of the teeth, which appeared in the 

 Palaeozoic, extends into the Triassic, 

 where it becomes extinct. These 



Fl< 1.404. Plesiosaurus macroccpna- 



animals varied in length from ten lus, restored. (After Owen, from 

 centimeters to several meters; the P^er and Hasweifs Text-book.) 



skull of one measured some sixty centimeters in breadth and 

 ninety in length, and the skin was provided with hard, bony 

 plates, such as are found in no living amphibia. 



