CRETACEOUS TYPES. 



129 



one. When, however, the molar teeth of the type in 

 which there are but two longitudinal rows of tubercles are 

 compared with the transitory teeth of the Australian duck- 

 bill, a certain resemblance can be detected between the two, 

 which seems sufficient to indicate that in Plagiaulax and 

 Tritylodon we have to do in all probability with ancient 

 types of egg-laying mammals. 



Till within the last few 

 years the Cretaceous period 

 formed a complete gap as 

 regards the history of 

 mammals ; and seeing that 

 in Europe, with the excep- 

 tion of the Wealden, the 

 rocks of this system are 

 mainly of marine origin, 

 while some of them, like 

 the chalk, were laid down 

 in seas of considerable 

 depth, this absence of 

 mammalian remains is not 

 to be wondered at. In the 

 United States the condition 

 of things is, however, very 

 different. There the upper- 

 most Cretaceous rocks are 

 of fresh- water origin, and 

 constitute a series known 

 as the Laramie, which is in 

 intimate connection with the lower part of the Tertiary, 

 and has yielded the extraordinary horned dinosaurs. From 

 these Laramie Cretaceous rocks Prof. Marsh has succeeded 

 in obtaining a quantity of teeth of mammals, although 

 these are, unfortunately, mostly found detached. These 

 teeth indicate mammals closely allied to Plagiaulax of the 

 Jurassic, and also others of a carnivorous type related to 

 Ampliifherium) or some of the many-molared carnivorous 

 forms from the Dorsetshire Purbeck. Mammals of the 

 Triconodont type- that is, those with the three cusps of 

 the molars in a straight line seem, however, by this time 



FIG. 43. Under part of tlie 

 Skull of a South African Secondary 

 Mammal. Two-thirds natural size. 



