PHYLETIC BRANCHES AMONG THE VERTEBRATES 171 



represented, from the upper Trias onwards by 

 the two much-differentiated types of the Shield 

 Turtles or Dermochelydse, ancestors of the existing 

 Sphargis, and of the pleuroderic Elodites, which 

 continues in the genus Podocnemis in the fresh 

 waters of South America. The Sphenodon of 

 New Zealand is to-day the sole representative of an 

 ancient branch of reptiles, the Rynchocephalians 

 very common in the shallow seas of the upper 

 Jurassic of our own country, and may be traced 

 with slight modifications as far as the Permian red 

 sandstone of Saxony. The Lacertians of the 

 Varans family are already represented in the lower 

 Cretacean of the isle of Lesina by a hardly distinct 

 form of the existing Hydrosaurus. We see clearly 

 by these examples that the longevity of certain 

 phyletic branches of the Fishes, Amphibians, and 

 Reptiles in no way yield the palm to that of the 

 majority of the branches of the Invertebrates. 



A demonstration as complete as this is awkward 

 to furnish in the actual state of the science, in the 

 case of the higher Vertebrates, and especially of the 

 Mammals. However, we know already that the 

 first origin of the mammalian trunk goes back 

 extremely far, and is very certainly earlier than the 

 upper Trias, the earliest stage in which there 

 has been noted the existence of true Mammals 

 coming within the normal type of structure of this 

 class. That Triassic Mammal, the Dromatherium 

 sylvestre of North Carolina, as far as can be judged 

 from the one half-mandible known till now, assimi- 

 lates closely enough to the type of the Marsupiales 

 polyprodontes (with more than four incisors in each 



