416 MESOZOIC TIME. 



Marsupial 1 near the modern genus Myrmecobius of Australia. 2 The 

 species has been named, by its discoverer, Dromatherium sylvestre. 

 Mammals of similar kinds probably spread over the continent, and may 

 have been of many species. 



Characteristic Species. 



1. Mollusks. — Lamtllibranchs. — Myacites Pennsylvanicus Conrad, from the 

 black slate of Phcenixville, Pa. Two other species occur at the same locality. 



In California or Nevada, are Oi-thoperas Blakei Gabb, Goniatites {Ammonites) levidor- 

 satus Hauer, Ceraiites (Goniatites) Haidinyen Hauer, C. Whitneyi Gabb, Ammonites 

 Blakei Gabb, A. Ausseanus Hauer, A. BiUinysianus Gabb, Halobia dubia (?) Gabb, 

 Monotis subcircukiris Gabb, Posidonomya stella Gabb, Myophoria alta Gabb, Spirifer 

 Homfrayi Gabb, besides other species. 



2. Articulates (a.) Crustaceans. — Ostracoids: Fig. 711, Estheria ovata Lea 



[Posidonia minuta), from Richmond, Va., and Phcenixville, Pa., resembles the P. 

 minuta of the European Trias; Fig. 711a, E.ovalis Emmons, from North Carolina, and 

 Fig. 711 b, E. parva Lea, Phcenixville, Pa., are both E. ovata, according to T. R. 

 Jones. Two species of Cypris, one smooth, and the other granulate, occur at Phcenix- 

 ville and Gwynned, Pa. Figs. 716, 717 represent tracks referred by Hitchcock to 

 Macrouran Crustaceans. 



(b.) Insects. — Fig. 712, exuvia of a Neuropterous larve, related to Ephemera, 

 according to J. L. Le Conte : the appendages along the sides are probably branchia? 

 attached to the abdomen. Tracks of different insects are shown in Figs. 713-715, from 

 Hitchcock. On comparing especially Figs. 713, 714 with the footprints of some living 

 Insects, Dr. Deane found a close resemblance between them. 



1 Mammals. — The highest group of Vertebrates are of two grand divisions : — 



I. The Ordinary or True Viviparous Mammals, such as the Monkey, Lion, Elephant, 

 Ox, Bat, Mouse, Whale, etc. 



II. The Semi-oviparons Mammals, which are, with one exception, Marsupial. — 

 Birth takes place before the ordinary degree of maturity in the embryo is attained, 



.and they thus approximate to oviparous vertebrates. The immature young in these 

 Marsupials are passed into a pouch (marsupium), situated over the venter of the mother, 

 in which they are nourished from her teats, until the degree of maturity required for 

 independent existence is attained. They are the lowest, and geologically the earliest,, 

 of Mammals. 



2 A view of the Myrmecobius is here given. 



Fig. 732. 



732, Myrmecobius fasciatus ( X 



