132 FROM FISH TO PHILOSOPHER 



ters in paleontology. The definitive mammalian tendency 

 in respect to skull and teeth is first evident in the Pely- 

 cosauria (pelykos = basin; sauros = lizard, so named in 

 reference to the form of the pelvis) of the Pennsylvanian, 

 indicating that the separation of the mammals from the 

 reptiles followed shortly after the evolution of the am- 

 niotic egg. More distinctively mammal-like forms appear 

 in the Permian Therapsida {therion = beast; apsides = 

 arch, so named because the temporal region of the skull 

 is like that in the mammal), which are known by two 

 hundred or more genera and an imknown number of 

 species, thousands of museum specimens and 'countless 

 thousands still in the rock.' It has been suggested, but 

 not proved, that the advanced therapsids, as represented 

 by the Theriodontia, may have been warm-blooded and 

 may have nursed their young. In the Cynodontia of the 

 Triassic the teeth had begun to acquire the characteris- 

 tic mammalian form of incisors, canines, premolars, and 

 molars, the roots of which were implanted in sockets. 

 Here mammaHan evolution divided into two streams, one 

 to produce the Monotremata or egg-laying mammals'— 

 Simpson calls these 'highly modified surviving therapsid 

 reptiles, mammals by definition rather than by ancestry' 

 —of which the duckbill (platypus), and the spiny ant- 

 eater (echidna), of Australia, are the only surviving 

 members, both so primitive that they lay shelled eggs 

 like their reptihan ancestors. 



The platypus is, in fact, one of nature's strangest mix- 

 tures and illustrates something of the transition from 

 the reptilian to the mammalian level. The typical mam- 

 mal is, by definition, a warm-blooded animal whose 

 body is covered with hair, and whose young mature 

 within the uterus and are bom alive and are nursed from 

 milk glands bearing nipples. The platypus, though cov- 

 ered with sleek mammalian hair resembling sealskin, lays 

 tough-skinned, reptilianHke eggs which it incubates with 

 its body, but the young are immature at birth (as with 



