23 
form of true bone. They present two kinds of surface markings— the 
concentric and the radiating— the concentric are ridges, which may be few 
or many; and the radiating are grooves, which in the ctenoid often divide 
the anterior implanted portion into lobes. The comb-like posterior part 
shows successive rows of prominences as the scale increases in size, but the 
surface lines are not the edges of successive layers as often described, but as 
Williamson has shown (Philosophical Transactions J or 1849 and 1851 J mere 
surface elevations of the upper layers. Vertical sections show that the 
scale consists of an upper and under part, of which the latter is by far the 
thickest. Both are divisible into lamina?, and are more or less consolidated by 
calcareous matter, which in the upper laminse of the under part occurs in 
the form of layers of oval concretions. The scales of the "lateral line" are 
of different shape to the rest, and generally more ossified. The scale of the 
Eel (Anguilla ) was selected as an aberrant form of considerable beauty. 
This scale has been differently described by authors, but Mr. Swayne 
considered that the circular or oval markings represent simply a peculiar 
modification of the ordinary surface pattern. 
In the Ganoid and Placoid — ganos, splendour ; plax, a broad plate — the 
scales are more organised, and consist of true bone or a modification called 
1 ' kosmine " closely allied to "dentine. ' ' The outer thin structureless layers are 
made of an enamel-like substance called Ganoin. The under part is traversed 
by delicate tubes called by Williamson "lepidine" 
The comprehension of the skin organs under the designation " dermoskeleton" 
seems most applicable in the Ganoid and Placoid orders. In the Palaeozoic 
fish the excess of calcareous matter in the exoskeleton was balanced 
by a diminution in the endoskeleton. In modern fishes several having bony 
scales are not included in the present order of Ganoids, which is divided into 
Lepido -ganoids and Placo-ganoids. The Lepido-ganoids — lepis, a scale — are 
divided into Cyclo -ganoids and Rhombo -ganoids, the latter having rhombic 
scales. The Rhombo-ganoids (now very few) were the most common in the 
Palaeozoic period. These had rows of scales mutually fastened by a peg 
projecting from one edge ; a great variety of forms have been observed. The 
Placo-ganoids include the sturgeons and the fossil Ostracostei. The sturgeons 
have five rows of overlapping oval bony scales with wrinkled surface. The 
Ostracostei (including Coccosteus, Pterichthys, &c.) had the forepart covered 
by a bony cuirass of broad, closely-fitting pieces ; the back part of the body 
was often undefended. The Trunk-fishes ( Ostracion ) seem to represent them 
at present. The Globe-fishes ( Diodon ) are covered by spines supported on a 
trifid base, which are capable of erection by inflation of the sk n. 
The Globe-fishes lead us to the Placoids, where the scales are in the form 
