246 MAMMALIA. 



It is as yet impossible to determine at what particular stage 

 in the evolution of the vertebrate skeleton the lung-breathers 

 first acquired the characteristic mammalian circulatory system, 

 the milk-producing glands, and a dermal covering of hair. It 

 is, however, clear that as soon as warm-blooded quadrupeds 

 were evolved the skeleton began to assume characters and 

 potentialities such as it had not exhibited before, and an 

 entirely new series of modifications became possible. In the 

 first place there was a simplification in the structure of the 

 skull. The postfrontal and prefrontal bones quickly disappeared, 

 probably by fusion with the frontals. The elements of the 

 single temporal arcade invariably became reduced by fusion 

 to two, a jugal in front, a squamosal behind ; and Palaeontology 

 seems to demonstrate (through the Anomodontia) that the 

 quadrate element, so characteristic of the lower vertebrates, is 

 included in the articular end of the hinder bone. The mandible 

 is also simplified, so that there is only one membrane-bone 

 in each ramus ensheathing the meckelian cartilage, which is 

 ossified and firmly united with its sheath at the articular end. 

 The well-formed limb-bones, 'adapted for support of the trunk, 

 exhibit a constant development of terminal epiphyses, already 

 noted in a less perfected degree in the Batrachia and in the 

 Sauropterygian and Chelonian reptiles; while this mode of 

 ossification has also extended to the centra of the vertebrae. In 

 the pectoral arch the coracoidal element soon disappears as 

 an insignificant process of the scapula. In the pelvic arch the 

 three bones of each side are always fused into an " innominate 

 bone " pierced by an obturator foramen. Except in the Cetacea 

 and rarely in certain Sirenia, the number of phalanges in the 

 digits is never multiplied beyond 2, 3, 3, 3, 3, however long 

 those digits may become ; and the ankle-joint occurs invariably 

 between the distal end of the crus and the tarsus, never 

 between the two rows of tarsal bones. 



This phase in the development of the vertebrate skeleton 

 was already very closely approached, if indeed not absolutely 

 attained, at the dawn of the Mesozoic period. It is exhibited 

 almost in its entirety by certain of the Anomodont reptiles 

 (especially the Theriodontia) in the Permian and Triassic ages ; 



