THE SCALES IN SOME TELEOSTBAN FISH. 53 
though each individual is mapped out by lines of coloration 
where the membrane persists in the interstices between the 
more elevated scalelets. The only reagent which I have as 
yet tried which seems to act readily upon the scalelets is 
hemalum, and this is not so marked as in the younger 
stages. 
Scalelets vary in shape and arrangement in different 
species of cod, more particularly in the lateral fields. The 
common cod and the coal fish furnish examples of the two 
most widely separated types. In the common cod (Fig. 6) the 
scalelets consist of a basal plate quadrilateral in shape. 
Those in or near the mesial line of the scale at either end are 
very nearly square, while in the lateral fields they are rhomboidal. 
Near to the peripheral margin of each scalelet there is a well- 
marked transverse ridge, which gives to it the appearance of 
a square envelope with a straight flap not fastened down, 
while the extreme peripheral and central margins are slightly 
bent upward, and are in contact with the adjacent margin of 
the scalelets immediately in front and behind.  ‘lowards 
the growing edge of the scale they are not always in such 
close contact, and a thin transverse line of colour between 
them may be seen in stained speciinens. The appearances 
are more readily understood in sections through the scale, 
and may be represented diagrammatically (Pl. VI, fig. 7). 
The same placoid pattern can still be recognised, but each 
scalelet is considerably expanded laterally. The transverse 
ridge near the peripheral margin is due to the centrally 
directed but minute spine; the anterior and posterior margins 
of the basal plate are upturned, the lateral being straight. 
The spines are most marked in the posterior field. The 
centrum or “ foyer” consists of a flattened plate of calcified 
material, oval in shape, with an irregularly serrated margin. 
From its appearance in section and on surface view and from 
the conformation of its margin, I believe it to be formed by the 
fusion of a number of basal plates, the spines of which have 
entirely disappeared. There is no indication of any markings 
such as would suggest that, previous to fusion, there had 
