DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
687 
Fig. 40 represents a vertical section of the articular portion of this bone, passing 
through the articulation towards the posterior inferior angle. The concave articular 
cavity is partly lined with a fibrous structure (40 a) and partly with true cartilage 
(40 b), the latter of these tissues being chiefly confined to the central and internal 
portions of the cavity. A thin film of chondriform bone (40 c, c') exists in the car- 
tilage along its line of junction with the osseous structure. 
Within the interior of this thick extremity of the bone is a large irregular cavity 
(40 d), which is of considerable width near the articulation, but contracts to a narrow 
space as it approaches the more flattened portions of the bone. It is traversed by a 
few thin and irregular laminae (40 e),and is bounded internally by others (40 /'),all of 
which are composed of raembraniform bone. Anteriorly, the latter terminate at an ob- 
tuse angle at the surface of the chondriform bone 40 c' ; corresponding in this respect 
with the relative positions of the two tissues in the condyle of the epitympanic (fig. .48), 
and differing only in the case of the opercular bone in the less oblique manner in 
which they come into contact. Similar lamellae exist in the upper part of the section 
(40 g) forming the outermost portions of the bone; only instead of terminating 
abruptly at the articular cavity, they bend downwards and inwards (40 g'), so that 
each additional growth not merely enlarges the area of the articulation, but adds to 
the thickness of its walls. No chondriform bone exists at this point, and the inflected 
lamellae are in contact with a fibrous tissue (40 a), instead of with cartilage. In the thin 
squamous portions of the bone, all the three series of lamellae, viz. the outer, middle 
and innermost, 40e,y’andg, are prolonged in parallel layers; those occupying the 
two free surfaces are extended as far as the several margins of the bone, whilst those 
which are internal appear to stop short of them, thus accounting for the cycloidal 
markings seen on the surface of the bone, which are really lines of growth. In 
the thicker portion of the structure, these membraniform lamellae are penetrated by 
curiously-formed Haversian canals (40 h). These do not however extend far into its 
thinner portions. The latter appear to receive their supply of nutritive fluids through 
numerous minute branching tubuli, which enter the bone at right angles from both 
its surfaces. They bear a close resemblance to those which are so abundant in 
the scales of Lepidosteus and other allied fish, and to which I gave the name of 
Lepidine. 
There can be little doubt but that the primary matrix upon which this interesting 
bone has been developed, has been the small portion of cartilage still existing at 40 b, 
but which, owing to the peculiar requirements of the articulation into which it enters, 
has never attained to any considerable size. Whether or not a prolongation from it 
once filled the large cavity 40 d is uncertain ; for though not improbable, I have seen 
nothing enabling me to conclude that it has done so. The existence of the thin film of 
chondriform bone at c, d, proves clearly that this is not merely a superadded articular 
cartilage, but a veritable part of the growing structure. The relations of the several 
parts shows that the genesis of this bone conforms, as to its type, with what we have 
4 T 2 
