102 COPE 



nect the cusps longitudinally or transversely, forming vari- 

 ous patterns, two of which are represented in Figs. 59 

 (page 124) and 63 (page 128). Such types are termed 

 lophodont. Those in which the crests and the grooves be- 

 tween them become respectively elevated and profound are 

 termed ptychodont, Figs. 51 (page 114) and 60 (page 125). 



Two subclasses are known to belong to the Mammalia : 



An interclavicle ; a large coracoid articulating 



with the sternum ; Prototheria. 



No interclavicle ; coracoid very small, coossified 



with scapula ; not reaching sternum ; Eutheria. 



The Prototheria have one existing order, the Monotre- 

 mata, and it is supposed, with much probability, that two 

 orders which appear in the Trias and continue until the 

 Eocene, inclusive, belong to it. If this classification is 

 admitted, the Eutheria have their first representatives in 

 the Postcretacic, and their latest are now numerous. 



Of the Pkototheria, there are probably three orders of 

 which species are known, but the location of the first two 

 enumerated below is not certain. 



Incisors reduced ; molars with compressed 



cutting crowns, and undivided roots ; Protodonta. 



Incisors much enlarged ; molars with tuber- 

 cular grinding surfaces, and distinct 

 roots ; Multituherculata. 



No teeth at maturity ; MonotremaCa. 



The families are the following : 



Protodonta ; Dromatheriidse. 



Multituherculata; Plagiaulacidse, Chirogidse, Polymastodon- 

 tidse. Monotremaia ; Ornithorhynchidse. 



