500 MESOZOIC TIME — REPTILIAN AGE. 



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

 zoic, 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-halj. 

 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 Jurassic of the first species 

 of the modern genus Crocodilus. 



(d.) The introduction of the class of Birds, in the Jurassic period, 

 if not, earlier, in the Triassic. 



(e.) Earliest Mammals, — Marsupials and Insectivores. — The introduc- 

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

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



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 



