VERTEBRATA. 



255 



more complicated development in socket?, that their 

 succession in time is more readily apparent than their 

 succession in place. Some fishes of this group have 

 also larger scales, sometimes armed with a protective 

 bony spine. These larger scales are also regarded, 

 like the larger teeth, as having been developed by 

 means of the fusion of the smaller ones. It will be 



Fig, 93. — Development of the embryonic tooth of a Newt. D, Dentine of 

 the tooth, formed in a papilla of the cutis. S; Enamel of the tooth. (From 

 Glaus and Sedgvfiok, after O. Hertwig.) 



seen further on, that those important little placoid 

 scales are not only regarded as having built up large 

 teeth and large scales, but also as having had, in the 

 course of the development of vertebrates, a little share 

 in the building up of bones. Although the name 

 placoid scale is still retained in books, it is a very 

 misleading one, because very few of these scales are 

 flat; they are often little bony knobs of very irre- 



