796 HISTORICAL GEOLOGY. 



Amphibians, and to typical numbers and arrangement in the bones of the leg, 

 has been stated on page 726. Once reached, these numbers remain the 

 normal or typical numbers for Reptiles, Birds, and Mammals. The typical 

 number of cervical vertebrae, seven, sometimes occurs in Reptiles ; but varia- 

 tion from this number is not in them a character of generic importance. 



Under Mammals, the differentiation of the teeth in all typical species, into 

 incisors, canines, and molars, exists, commencing with Triassic Marsupials ; 

 but the number of teeth continues to be multiplicate through the Jurassic, 

 the typical Mammalian number, 44, being usually exceeded, and sometimes 

 by 24. The niimber seven became the fixed or normal number of cervical 

 vertebrae, first, among Vertebrates, in Mammals. It is a character of all 

 existing Marsupials, and probably was of those of the early Mesozoic, — a 

 doubt remaining because no skeleton of an ancient species has yet been 

 found. 



Exceptions to normal numbers, after they were once attained, have pro- 

 ceeded from specializations in the course of upward as well as downward 

 progress ; but the larger part occur among degenerate forms, and in these, as 

 the examples mentioned show, the divergence is often very great. 



2. Location of the function of locomotion. — As remarked on page 726, the 

 typical Amphibian, on becoming adult, passes from the stage of caudal or 

 urostlienic locomotion, to locomotion by limbs, ov podosthenic. The latter is 

 the typical condition in Reptiles, Birds, and Mammals. But groups under 

 each differ as to the pair of limbs which bears the chief part of the work. 



The Triassic and Jurassic periods were distinguished eminently by 

 hind-limb location of force and locomotion. It was the era of very small 

 brains, and of great development of the posterior extremities — the era of 

 Merosthenic Vertebrates, as the Devonian and Carboniferous eras were of 

 Urosthenic Vertebrates. The prominent feature of all Dinosaurs is their 

 enormous hinder parts. Moreover, as has been mentioned, many of the 

 species, the gigantic Stegosaurs preeminently, have a provision for this 

 arrangement of the forces of the Reptile, as Marsh first brought out, in the 

 great nervous mass of the sacrum. 



The Amphibians also were strongest in the hind limbs, as is indicated by 

 the remains of the Labyrinthodonts. The wings of the Jurassic Bird of Solen- 

 hofen prove that they were poor flyers, and consequently that their legs or 

 hind limbs were their chief locomotive organs. Moreover, in this merosthenic 

 era, the Mammals probably had the hind limbs much the stronger of the two 

 pairs, as is true of modern Marsupials. 



The species of Reptiles that were distinctively strong in the fore limbs, or 

 prosthenic, are the Pterosaurs ; and among these, the Pterodactyls, having 

 the head large, the posterior feet small, and the tail short, with the brain 

 and sternum Bird-like, appear to have taken the lead. Seeley has placed 

 them in an independent group separate from Reptiles, The absence of scales 

 from the body, and the light bones, with air cavities and pneumatic foramina, 

 still further ally them to Birds, and separate them from other Reptiles. It 



