MESOZOIC TIME — TRIASSIC AND JURASSIC. 795 



three species exist, one of Oriiithorhynchus and two of Echidna) are the 

 lowest of Mammals, and have many Amphibian and Reptilian characters in 

 their skeleton, besides that striking one of bearing eggs, like E-eptiles and 

 Birds. They are called Prototheria in some zoological systems ; and this 

 they undoubtedly are in type, though the Duck-like bill and webbed foot of 

 the Ornithorhynchus are unquestionably degenerate characteristics ; for the 

 earliest species had almost certainly a full set of teeth. That they were 

 first in origin, however, is far from proved. 



Among Reptiles, the Permian type represented by the genus Palmohat- 

 teria, with the associated RhyncliocephaUa, as explained on page 707, is the 

 most generalized or comprehensive of the class. Besides its Amphibian rela- 

 tions on one side and its Reptilian on the other, it has, as Baur explains, 

 characteristics also of Birds and Mammals. This author regards the type 

 as the precursor type of the class of Reptiles and also of the class of Birds. 

 It is like Mammals, he states, and unlike all other Reptiles, except the Rhyn- 

 chocephs, in having the foramen of the distal end of the humerus on the 

 inner side of the epicondyle ; in other Reptiles it is on the outer side or is 

 absent ; and it is absent from all Amphibians and Birds. It is probable, 

 therefore, that, nearly as Baur concludes, the line from the Amphibians 

 which gave off a Rhynchocephalian branch, later gave off a Mammalian. 



The relation of Birds to the Dinosaurs in pelvis and hind limbs, especially 

 to the Carnivorous kinds, was pointed out by Huxley ; and it is supposed 

 that the two types may have originated from a common type in either the 

 Triassic or Permian period. The Jurassic bird, Archaeopteryx, which is so 

 remarkably Reptilian, has the long limbs, and but little else, of a Dinosaur ; 

 and this feature in the hind limbs of both is partly a consequence of an 

 elongation of the metatarsals. The cranium and the steriuim are Bird-like, 

 but not so the fore limbs, pelvis, and some other parts. The Berlin specimen 

 was first described as a Reptile by Carl Vogt. The relations of Birds to 

 Dinosaurs in the structure of the skeleton are largely a consequence of the 

 demands made by the animal on its hind limbs ; and the unlike demands on 

 the fore limbs are the source of divergences. 



General Changes Attending Biological Progress. 



1. Reduction in midtiplicate numbers. — The reduction in number of pos- 

 terior vertebrae when the Fish type passed to that of the Amphibian has been 

 noticed on page 726. Their absence from the upper lobe of the tail in most 

 Triassic Ganoids, rendering the Pish homocercal in place of heterocercal, is a 

 change in the same direction, like that which takes place when the Tadpole 

 becomes a Frog, or the young of a Ganoid or other Pish loses a caudal lobe, 

 or some caudal vertebrae, when becoming adult. The long vertebrated tail of 

 the Jurassic Bird was a related multiplicate feature, which disappeared early 

 in Cretaceous time, if not before it. 



The reduction of the number of parts in the limbs of Pishes before the 

 close of the Paleozoic to the typical number of five for the digits in 



