IV. VERTEBRATA 



451 



band, following the contour of the plate. The nail (laimia) is nearly flat, and 

 since the sole is reduced, the nail appears as a purely dorsal structure [11). 



Of great importance in understanding the dermal ossifications is the 

 fact that all scales of fishes are derived from the placoid scale of the selach- 

 ians. These are rhombic plates, bearing in the middle pointed spines, 

 called dermal teeth from similarity in structure and development to the 

 teeth of the mouth cavity (fig. 510). They consist of dentine (d) and 

 have a large pulp cavity {p), with numerous blood-vessels in the interior. 



Fig. 509. Fig. 510. 



Fig. 508. — Long sections through the toes of /, horse; //, ape; //, dog. ft, ball of 

 toe; />, claw plate; s, sole plate; w, root of claw; 2, 3, second and third phalanges. 



Fig. 509. — Sections (schematic) of scales of reptiles. /, snake; //, 'blind worm.' 

 I, corneum; 2, Malpighii; 3, corium; 4, bony scale. 



Fig. 510. — Sagittal section of a scale of Scyttium stellare (after Hofer). b, basal 

 plate; d, dentine; p, pulp cavity; sch, enamel. 



Whether the thin layer (scJi) covering the tip can be called enamel is dis- 

 puted. Dermal teeth and true teeth are identical structures which, from 

 different position and consequent difference of function, have developed 

 differently. 



The scales of fishes have a further anatomical interest, since from them 

 have arisen, besides the bony plates which form the armor of the turtles, 

 alligators, and many mammals (armadillos), important parts of the axial 

 skeleton, the secondary or membrane bones. A membrane bone is a bony 



