170 



COMPARATIVE ANATOMY 



Among vertebrates, the placoid scales, which first appear in certain 

 sharks of the Upper Devonian, are especially important because of their 

 further evolution. Each of these scales has a flat basal plate of dentine 

 embedded in the skin, and each has commonly also a projecting spine 

 coated with hard enamel like a tooth. From these minute placoid scales 

 of ancient sharks have evolved all the multiform teeth of all the higher 

 vertebrates. 



From these and other types of scale have evolved also, by simple 

 enlargement, the heavy continuous dermal armor of ganoids and other 



^^^^ 



•ENAMEL SPINE. 



Fig. 128. — The imbricated pattern of placoid scale arrangement in elasmobranchs. 

 The scales are arranged in rows and usually each scale is in line with the interval between 

 scales of the lines in front and behind. (Redrawn after Klaatsch.) 



fishes. These same bony plates survive also in man and the higher 

 vertebrates as "membrane bones" which, unlike most parts of the skeleton 

 are not pre-formed in cartilage but develop directly in connective tissue, 



HORNY SCALES 



Vertebrates, besides bony scales, have also horny; but these have 

 played a much less important part in evolution, and are confined to 

 amniotes, more especially, reptiles. 



In reptiles, the stratum corneum forms a continuous scaly layer over 

 the entire body, the separate scales being local thickenings which con- 

 tinue to grow by the addition of new keratin from underneath. Serpents 

 commonly shed this scaly coat twice a year. But the rattlesnake retains 



