52 HALF-AN-HOUR AT THE MICROSCOPE. 



" glittering sheen " of the scale-armour. With the lowest power 

 (the 4-in. glass) a good general idea is obtained ; the scales then 

 have a general resemblance to those of some fossil fish. View it 

 as an opaque, then as a transparent object, and see how very 

 different it appears. Then put on the 2 in. glass — with a general 

 resemblance see the individual difference between all the scales, 

 no two are alike — so that after a little practice the eye can readily 

 separate any one, and discriminate its special characters. With 

 the Yi object-glass and "A" eye-piece, observe the surface of 

 particular scales first, with their ridges increasing in number and 

 elevation as the scale becomes larger and (we must suppose it) 

 older. An arch of lighter colour, probably a canal, can be seen 

 internally, stretching forwards between the first lateral ridges, with 

 a branching canal on either side, passing from the arch to the 

 cusps. Through these nutriment is conveyed ; life and continued 

 growth are rendered possible. Turn the slide, so that right and 

 left are reversed, and see how wonderfully like a tooth is each 

 scale ! There are fossil-shark's teeth which, in their natural size, 

 closely resemble these scales when magnified to fifty diameters. 

 Now, by focussing down, a view is obtained of their mode of 

 implantation into the skin, by processes resembling for each scale 

 a star of four rays. And see how closely the scales are inter- 

 locked ! each has six others to support it. The small scattered 

 spines of the Thornback skate are supported by processes forming 

 a i'zJv-rayed star. Placed at a distance over the surface 

 of the skin, they would appear to require, each individually, 

 the firmer hold gained by having six roots instead of four. Now, 

 supposing what we have before us, after lying in water sufficiently 

 long to decompose the skin and softer parts, and separate the 

 scales — that the outer, harder portions of the latter become 

 fossilised. What is there then to distinguish the so tooth-like 

 scales from teeth proper ? Absolutely nothing ! The careful 

 study of the context can alone prevent grave errors from being 

 fallen into. T. P. Barkas, in his admirable monograph on coal- 

 measure (Palreontology) has a figure (158, pi. 4), respecting which 

 he says (page 44 of text) : " I have considerable doubt respecting 

 their dental character ; they may be dermal spines rather than oral 

 appendages." And on looking at a section of " Tooth of 

 Ctenoptychius," in a set he has kindly allowed me to inspect, 

 I am so struck by the absolute resemblance between the micro- 

 scopic characters of this so-called " Tooth " and some specimens 

 now well recognised as being dermal scales (though formerly 

 described as teeth !) that I turn to Barkas for information, and there 

 find under " Ctenoptychius" : — It is assumed that these fossils are 

 teeth, or true oral appendages, but so far as my observation has 

 gone, I have not found any in consecutive order. They may be 



