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. — Lamellibranchs. — Myacites Pennsylvanicus Conrad, from the 

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



In California or Nevada, are Oriho/ceras Blakei Gabb, Goniatites {Ammonites) levidor- 

 satus Hauer, Ceratites (Goniatites) Haidingeri Hauer, C Whitneyi Gabb, Ammonites 

 Blakei Gabb, A. Ausseanus Hauer, A. BilUngsianus Gabb, Ilalobia dubia (?) Gabb, 

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

 Homfrayi Gabb, besides other species. 



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

 (Posidonia minuta), from Richmond, Va. ; and Phoenixville, Pa., resembles the P. 

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

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

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

 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 branchiae 

 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-oviparous 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>£)- 



