LIFE IN TERTIARY TIMES 287 



took place in such a manner that we can pass consecutively 

 from primitive to modern forms, each fossil genus furnishing 

 us with a link in the chain binding them together. A great 

 many series remain outside this chain. They are like the 

 branches of independent genealogical trees, forming a veritable 

 forest when seen from above, but in which it is extremely 

 difficult to recognize the trees, and, on these trees, the branches 

 to which present-day forms should be attached. 



The small Mammalian forms that appeared in the Trias 

 (Tritylodon) are encountered again with their multituberculate 

 molars and complete coracoid in the form of Neoplagiaulax, 

 Polymastodon, Ptilodon, and Chirox, in the Nummulitic 

 deposits of New Mexico. 1 The Ornithorhynchus and the 

 Echidna are their present representatives. The horny tooth 

 of Ornithorhyncus is, in fact, preceded by a rudimentary 

 indication of a multituberculate tooth. These Mammals, 

 confined to New Guinea, Australia, 2 and New Zealand, even 

 in some cases to Australia alone, 3 are still oviparous, and it 

 is probable that the multituberculate forms also were oviparous. 

 These Mammals, which form the sub-class Monotremata or 

 Prototheria, are removed only from Reptiles by their hairy 

 skin richly provided with glands, some of which are already 

 lactigenous ; their mode of reproduction, the structure of the 

 shoulder-girdle comprising two clavicles united into a sort of 

 fork, with a coracoid and a precoracoid bone on each side, like 

 that of the Lizards, and their marsupial bones, the last remnants 

 of abdominal ribs, are distinctly reptilian. 



With the exception of the Theriodontia, the Reptiles were 

 chiefly modified in the direction of locomotion, for they retained 

 their simple teeth, with a cutting edge in plant-eaters and 

 sharp points in flesh-eaters. The Mammals, on the contrary, 

 evolved in three directions — gestation, dentition, and locomo- 

 tion. Further, their flexible skin, permanently moist, and 

 rich in glands, but also in the sensitive cells to which even 

 hairs are only, as it were, annexed, constituted a source of 

 multiple excitation, which explains in some measure the rapid 

 development made by their cerebral apparatus. 



From the point of view of gestation the present viviparous 



1 In the San Juan Valley (Puerco and Torrejon beds). 



2 Proechidna and Echidna. 



3 Ornithorhynchus. 



