1907.] THE SCALES OF FISH. 757 



and pulp-cavities at the periphery (text-fig. 197, b). The cosmine 

 layer with its thin enamel-like covering, is formed once and for all 

 when it is first laid down. As far as one can judge from the 

 examination of sections it does not materially alter with age. No 

 new layers are deposited above it ; such changes as take place are 

 unimportant, and chiefly due to the filling up of the various 

 spaces. The stratification of the isopedine indicates the lines of 

 growth. Faint signs of laminte are also visible in the walls of the 

 vascular spaces. 



We may summai-ise the characters of the cosmoid scale as 

 follows : — It has an outer layer of dentine-like substance with 

 pulp-cavities and vascular chambers arranged in regular manner ; 

 a middle bony layer with vascular spaces ; and an inner layer of 

 bony laminte, pi-obably ossified fibrous tissue of the cutis. The 

 cosmoid scale grows in thickness only by the addition of new 

 lamellae below ; its outer surface is covered with a thin shiny layer,, 

 the nature and origin of which is uncertain. 



The Ganoid Scale. — It is proposed to restrict the name "ganoid" 

 to a type of scale found in all the Actinopterygii except the modern 

 Teleostei. In its full development this type is represented by the 

 rhombic scales of Pcdceoniscus and Lepidosteus. It difiers radically 

 from the cosmoid scale described above in that it grows in 

 thickness by the addition of new layers not only below, but also 

 on its upper surface. In fact concentric layers of new substance 

 are continually being deposited over the whole surface ; the oldest 

 part of the scale is' therefore at the centre. These layers, 

 however, are not the same throughout ; the lower being bony or 

 fibrous, the upper of much denser homogeneous, enamel-like 

 substance called ganoine by Williamson (text-fig. 198, c). 



There are two distinct varieties of "ganoid" scale, differing in 

 constant and important characters : — 



The Palceoniscoid Scale. — Gonatodus or Eurynotus yields good 

 examples of this type. The exposed surface of the scale is covered 

 with a shiny layer of ganoine, pierced here and there by small 

 vascular canals leading downwards into a horizontal network of 

 canals (PI. XLY. tig. 15, & text-fig. 198). From this again a few 

 vertical canals pass downwards to open on the lower surface. The 

 bulk of the scale, below the network of vascular spaces, is made up 

 of the usual horizontal laminse of bone, arranged in parallel layers. 

 At the periphery these layers are bent upwards and, so to speak, 

 turned over to form the laminse of ganoine covering the outside of 

 the scale. The two are continuou.s ; but at their junction, just 

 above the vascular spaces, is a cosmine-like layer penetrated by 

 bunches of minute branching canaliculi passing upwards (fig. 15). 

 Bone-cells are abundant below the vascular network, but none 

 are found above it. Thus, in the Palseoniscoid scale, a layer of 

 cosmine is interposed between the lower bony and the upper ganoine 

 layers. 



