DR, A. GUNTHER’S DESCRIPTION OF CERATODUS. 
517 
membranous laminae. The outer stratum (fig. 4, d) is calcified, the inorganic matter 
consisting chiefly of phosphate of lime, and in much less quantity of carbonate of 
lime. The thinnest vertical and horizontal sections give but an indistinct image of the 
arrangement of its constituent tissues ; the greater portion is composed of a great num- 
ber of transparent calcareous globules, the form of which is less regular towards the 
surface, which is studded with conical prominences ; and these prominences are con- 
stantly reproduced from the underlying globules, of which I have seen several assuming 
a conical form (fig. 4, d). When we destroy the organic matter by burning, the scale 
breaks up into the smaller divisions indicated by the superficial sutures. In the central 
parts the surface of the calcified matter (fig. 6) is then found to be raised into numerous 
conical spines, the base of each spine being surrounded by small foramina*. In the 
marginal and basal portions of the scale (fig. 8) the spines are placed on ridges running 
parallel with the longitudinal axis of the scale, and separated from one another by valleys. 
The bottom of these valleys is perforated like a sieve. The spines are more densely 
placed on the basal part of the scale than on the sides, and are entirely absent on its 
exposed portion (fig. 7), where the ridges run at a right angle to the free border of the 
scale, and are frequently connected with each other by transverse ridges. 
In the scales of Protopterus cmnectens the outer stratum contains but a small quantity 
of salts of lime, which form a very thin sieve-like lamina. 
In no part of the scales of Ceratodus have I discovered bone-corpuscles, which is very 
singular, inasmuch as Kolliker has not only found them in Lejyidosiren , but also in 
those fishes the bones of which are provided with those corpuscles. 
Nasal Cavity. 
As in Lepidosiren, there are two nasal openings on each side (Plate XXXIV. fig. 3, n), 
both being situated within the cavity of the mouth, the anterior somewhat in front of 
the vomerine teeth, and the posterior outside of the front part of the molar. The nasal 
cavity is coated with the transversely folded pituitary membrane, the transverse folds 
being divided by one longitudinal fold ; there are about twenty-three transverse folds. 
Eye, 
The eyeball has a transverse diameter of 13, and a longitudinal one of 10 millims. 
The optic nerve perforates it somewhat out of its axis. The pupil is circular; the lens 
spherical, and not steadied by a falciform process. Also the choroid gland is absent, as 
in Polypterus and Lepridosteus. The uvea is of a deep black colour, but leaves a great 
portion round the entrance of the optic nerve uncovered. The sclerotic capsule is 
strengthened by a single cartilaginous spheroid, which, in a horizontal section made 
through the greatest periphery of the eyeball, is three fourths of a millimetre thick. 
The Organ of Hearing , being enclosed in the cranial cavity, will be described in con- 
nexion with the skull (p. 526). 
* Resembling the sculpture of a shield of Aslerolejpis. 
