516 
DR. A. GUNTHER’ S DESCRIPTION OE CEEATODUS. 
base of the exposed portion. From the head to opposite the vent there are 22-23 scales 
in the lateral line ; thence the size of the scales diminishes, and about seventeen more 
scales may be counted in the continuation of the lateral line. So far our description 
applies to all the specimens known at present ; but with regard to the number of longi- 
tudinal series of scales, a remarkable difference exists between the examples first known 
and the two others obtained at a later period from the Mary river. 
In the former the middle of the trunk is surrounded by eighteen series of scales, five 
of which are above, and eleven below the lateral lines. This form has been obtained 
from the Burnett river; and the type of Ceratodus forsteri belongs to it. 
In the second the scales are conspicuously smaller, and their external sculpture is less 
intricate than in the former. The middle of the trunk is surrounded by twenty-one 
series of scales, six of which are above, and thirteen below the lateral lines. This form 
may be named Ceratodus miolepis. 
Structure of the Scales (Plate XXXI.). 
In an example of C. forsteri 38 inches long, a scale taken from the middle of the side 
is 2§ inches long and If inch broad (figs. 1 & 2). Anteriorly its outline is rounded, the 
lateral margins being almost parallel, and meeting the hind margin at nearly a right angle. 
The exposed part of the scale is not more than about a fourth of the entire surface of 
the scale, and is covered with a brownish-black membrane. The outer surface, especially 
of the imbedded portion, is rough, the inner perfectly smooth. Four areas can be dis- 
tinguished on the outer surface : — 1, the exposed portion, covered with a thin brownish- 
black membrane ; 2 and 3, the lateral areas of the imbedded portion, divided into oblique 
rows of trapezes arranged like scales of a Ganoid fish ; and, 4, a median area, triangular 
in shape, extending from the centre of the scale to its posterior angles, and irregularly 
longitudinally striped. The inner surface (fig. 2) is quite smooth, but furrowed by lines 
indicating the course of blood-vessels, with which the membrane of the pouch of the 
scale is provided ; they form an irregular network on the exposed and median areas, 
densely distributed on the former and sparingly on the latter, and follow the sutures 
between the trapezes of the lateral areas. The inner surface of the exposed portion 
shows also numerous pores by which vessels enter the interior of the scale. 
The microscopic examination of the scale reveals the following structure*. In a ver- 
tical section (figs. 3 & 4) it is shown that each scale is composed of an inner thicker, 
and an outer thinner and harder stratum, The former consists of numerous layers of 
fibrous cartilage (“ membranous laminae” of Williamson), more numerous in the centre 
of the scale (I count about forty-two), the number of layers decreasing towards the 
margins. The fibres of one layer run parallel to one another, but generally cross those 
of the next layer at an angle of either 90 or 45 degrees (figs. 4 & 5). I have not found 
any of the isolated lenticular calcareous bodies which Williamson saw imbedded in the 
* For comparison with the structure of other cycloid scales I refer to Williamson’s original researches, in 
Phil. Trans. 1851, p. G47. 
