406 ORDERS OF MAMMALIA. 



Mammalian remains have hitherto been found ; but in the fresh- 

 water formation of the Middle Purbeck, at the top, namely, of 

 the Oolitic series, as many as fourteen small Mammals have 

 been discovered. These constitute the genera Plagiaulax, 

 Spalacotherium, Triconodon, and Galestes. Another gap then 

 follows, no Mammal having hitherto been discovered in any 

 portion of the Cretaceous series (with doubtful exceptions). 



Leaving the Mesozoic and entering upon the Kainozoic 

 period, remains of Mammals are never absent from any of the 

 geological formations. From the base of the Eocene Rocks 

 up to the present day remains of Mammals commonly occur, 

 constantly increasing in number and importance, till we arrive 

 at the fauna now in existence upon the globe. 



In the following are given the characters of each order of 

 the Mammalia, with the range in time, and, so far as known, 

 the more important fossil forms of each. The number, how- 

 ever, of known fossil Mammals is so great, and in many cases 

 they exhibit so many peculiarities and divergences from exist- 

 ing forms, that nothing more can be attempted here than to 

 give a brief and general sketch of the palaeontological history of 

 the class ; attention being drawn, where it may seem necessary, 

 to extinct types of special interest. 



CHAPTER XXXVI. 



ORDERS OF MAMMALIA. 



ORDER I. MONOTREMATA. The first and lowest order of the 

 Mammalia is that of the Moiiotremata, containing only two 

 genera, both belonging to Australia namely, the Duck-mole 

 (Ornithorhynchus) and the Porcupine Ant-eater (Echidna]. 



The order is distinguished by the following characters : 

 The intestine opens into a " cloaca," which receives also the 

 products of the urinary and generative organs, which discharge 

 themselves into a urogenital canal the condition of parts 

 being very much the same as in Birds. The jaws are either 

 wholly destitute of teeth (Echidna), or are furnished with horny 

 plates which act as teeth. The pectoral arch has some highly 

 bird-like characters, the most important of these being the 

 extension of the coracoid bones to the anterior end of the 

 sternum. The females possess no marsupial pouch, but the 

 pelvis is furnished with the so-called " marsupial bones," be- 



