28 FAMILY ^ 



thorns of the ray (fig. 2, a) exemplify the extreme of the 



prickly type ; the fins, abdomen, and anterior part of the 



head of the spotted dog-fish {Scyllium stellare) are covered 



by lozenge-shaped little plates, which glisten with enamel, 



and are so thickly set that they cover the entire surface of 



the skin (fig. 3, h) ; and these seem equally illustrative of the 



scale-like form. They are shagreen points passing into osseous 



scales, without, however, becoming really such ; though they 



approach them so nearly in the shape and disposition of their 



upper discs, that the true scales, also osseous, of the Acan- 



thodes sulcatus (fig. 3, a), a ganoid of the Coal Measures, 



can scarce be distinguished from 

 rig. 3. , 1 . . ,, 



them, even when microscopically 



examined. It is only when seen in 

 section that the distinctive differ- 

 ence appears. The true scale of the 

 Acanth, though considerably elevat- 

 ed in the centre, seems to have been 

 planted on the skin ; whereas the 

 scale-like shagreen of the dog-fish is 



elevated over it on an osseous pedicle 

 a. Scales of Acanthodes ml- r...n//^r.\ i 



catus. ^^ lootstalk (tig. 0, a), as a mushroom 



I. Shagreen of Scyllium stel- is elevated over the sward on its 

 lare. (Snout). i i i /. , ,i . 



^^ . w T J. V stem : and the base of the stalk is 

 (Mag. eight diameters.) 



found to resemble in its stellate cha 



racter that of a shagreen point of the prickly type. The ap- 

 parent scale is, we find, a bony prickle bent at right angles 

 a little over its base, and flattened into a rhombodial disc 

 atop. 



In small fragments of shagreen (fig. 2, h), which have been 

 detected in the bone-bed of the Upper Ludlow Rocks (Upper 

 Silurian), and constitute the most ancient portions of this 

 substance known to the palaeontologist, the osseous tubercles 

 are, as in the minuter spikes of the ray, of the upright thorn- 



