500 MESOZOIC TIME — REPTILIAN AGE. 



The number of non-siphonated (or integripallial) Conchifers in the Meso- 

 Eoic, discovered up to 1849, according to a computation made by Bronn, was 

 2358, and of siphonated (or sinupallial), 1089; making the former over twice 

 as numerous as the latter. Bronn gives for the corresponding numbers for 

 living species 1480 and 1190. Of the integripallial Mesozoic species, one-half 

 were Pleuroconchs (species having unequal valves), while in modern seas only 

 about one-fourth are Pleuroconchs, the Orthoconchs amounting to 73 per cent. 



4. Among Articulates. — (a.) Crustaceans. — The rise of the class of 

 Crustaceans from Macrourans (Shrimps and Lobsters) to true Crabs. 

 These Crabs belong to the division of square Crabs (or the Grap- 

 soids) ; the higher divisions, the Cancer type (arched front) and 

 Maia type (triangular Crabs), are still unrepresented. 



(b.) Insects. — The unfolding of the class of Insects, nearly all the 

 tribes being present in the later Jurassic, — even the highest tribe 

 of Hymenopters, in the form of a species related to the Bee. 



5. Among Vertebrates. — (a.) Teliost Fishes. — The introduction in the 

 Cretaceous of the grand tribe of Teliosts, or common osseous fishes ; 

 and among the early representatives of the tribe there were species 

 related to the Salmon, Herring, and Perch. 



(b.) Selachians. — The introduction in the Jurassic of the modern 

 tribe of Sharks, — the Squalodonts (genera Sphenodus, Lamna, Oxy- 

 rhina, Notidanus, etc.). 



(c.) Crocodiles. — The appearance in the Cretaceous of the first 

 species of the modern genus Crocodilus. 



{d.)Earliest Mammals, — Marsupials and Insectivores. — The introduction 

 in the Triassic of the earliest of Mammals, and their increase in 

 genera in the Jurassic, — the species, Marsupials and Insectivores. 



(e.) Cetaceans, or Whales. — The introduction in the Cretaceous of 

 the earliest Mammals of the tribe of Cetaceans, or Whales. 



4. System in the Progress of Life. 



Comprehensive Types. — Next to the Labyrinthodonts, remarked 

 upon on p. 395, the Cycads are the most marked example of a com- 

 prehensive type in the Mesozoic. These plants — the characteristic 

 species of the era — are related in some fundamental points both to 

 the Ferns and Palms. They are like the former in that the leaves 

 are rolled into a coil in the bud, and unroll on expanding ; they 

 resemble the latter in the form of their foliage and in the general 

 habit of the plant. The first was a retrospective feature, for the 

 Ferns were of Palseozoic origin ; the latter was prospective, the 

 Palms not having yet appeared. 



The Marsupials are another example under this head. They 



