92 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



be noted that most of the membrane bones of the skull (infra} 

 belong in this category. Here, too, may be enumerated feathers, 

 hair, horn, claws, etc., as well as cornifications of the skin of 

 more limited distribution. 



Scales. The most primitive type of this exoskeleton is 

 found in the scales of the elasmobranchs. Here papillae of the 

 derma (dentinal papiljae), arranged in quincunx, push up into 

 the epidermis, carrying the basal layer of the latter before them. 

 The external surface of each papilla and its base secretes a little 

 plate of bone or dentine with a central spine ; while the epi- 

 dermis covering the papilla becomes converted into an enamel 



FIG. 98. Developing scales of dogfish (Acanthws~). b, basal layer of ecto- 

 derm; c. derma (corium) ; d, dentine; e, enamel; eo, enamel organ; /, pulp. 



organ, the deeper face of which secretes a hard enamel cap upon 

 the dentine base, the enamel being thickest upon the central 

 spine. These scales are known as placoid scales, and in their 

 development they show the closest similarity with teeth (see 

 p. 19). 



In the ganoids (JLepidosteus} the early development is as in 

 elasmobranchs, including the formation of plate, spines, and a 

 rudimentary enamel cap. Later the spines and enamel cap dis- 

 appear, while the outer side of the dentinal plate becomes cov- 

 ered by a hard, smooth layer known as ganoin, which differs 

 from enamel in that it arises from the derma. In the higher 

 ganoids and in the teleosts, dentinal papillae are formed ; but the 

 resulting scales are entirely of dermal origin, and, whether soft 

 and flexible, hard and bony, show no differentiation into layers. 

 At first these scales are arranged in quincunx parallel to the 



