484 



MESOZOIC TIME. 



folded, even to its highest tribe, that of Hymenopters, species of this 

 group, related to the bee, having occurred. 



Vertebrates. — Fishes. — Ganoids and Selachians (or fishes of the 

 Shark tribe) continued to be predominant kinds through the era ; but 

 the higher type of Teliosts (or common osseous fishes) appeared in 

 the Jurassic, and included many species in the Cretaceous. 



The Ganoids lost, in the Triassic, the Paleozoic feature of verte- 

 brated tails ; and this is a mark of progress ; for it is an example of 

 that abbreviation of the posterior extremity which generally marks 

 elevation in grade as well as progress in embryonic development. In 

 the Jurassic period, the number of species of Ganoids reached its 

 maximum, and also the diversity of generic forms ; and this therefore 

 was their period of culmination. The Ganoids are at present nearly 

 an extinct tribe. 



The tribe of Sharks was numerously represented by large species 

 of the Hybodont, Cestraciont, and other groups ; and the family of 

 Cestracionts, those having a pavement of bony pieces in the mouth, 

 for mastication, appears to have passed its maximum in the Creta- 

 ceous : it is now nearly extinct. But the highest species of the Sela- 

 chians existed later in the Tertiary. 



Amphibians. — The; scale-covered Amphibians, called Labyrintho- 

 donts, which first appeared in the Carboniferous age, had gigantic 

 species in the Triassic, and none afterward, so far as known. The type 

 of Amphibians therefore culminated in the Triassic, the Labyrintho- 

 donts being its highest species. We have now among Amphibians 

 only the naked-skinned Frogs and Salamanders. 



Reptiles. — All the grand divisions of this class were displayed, 

 and the type culminated in this era. 



The Enaliosaurs or Swimming Saurians of the Triassic — the 

 Nothosaur type — had the open skull of a Batrachian ; but, in the 

 Jurassic, the group rose to the higher grade of the Ichthyosaurs and 

 Plesiosaurs ; and there were several genera and numerous species : 

 with the Cretaceous, the species disappeared. 



The Lacertians commenced in the Permian, in species with the 

 ichthyic characteristic of biconcave vertebras, and retained it through 

 the Triassic and also in some Jurassic species. In the Triassic, 

 Jurassic and Cretaceous, Crocodilians existed in many species of great 

 size, and after the Triassic, some were without this low feature. 



Snake-like Reptiles occurred, of enormous size, in the Mosasaur 

 tribe, and with articulated jaws, precursors of our modern smaller 

 snakes. 



The Saurian type in the Jurassic rose to the grade of Dinosaurs, 

 the highest in rank, and among the largest of Reptiles ; and these all 

 disappeared, by or soon after the close of the Cretaceous period. 



