694 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
this process in the bones of the Eel, they appear to conform to the leading phenomena 
seen in those of the Perch and its allies. The condyles of the epitympanic, for ex- 
ample, exhibit a very similar distribution of the cartilage and the bone, both chon- 
driform and rnembraniform, as in those of the Perch. The lacunae in the membra- 
niform lamellae appear to be merely additional appendages to the structure, and in no 
way influenced by the relations between the bone and the cartilage. In the vertebrae 
of the same fish we see no trace of cartilage, and yet the lacunae are abundant ; their 
plane and that of their spreading canaliculi following the direction of the contiguous 
flexed lamellae, between which they are grouped. It is manifest that they are mere 
spaces which were left open when these lamellae were successively added to the various 
surfaces of the growing bone, and are perfectly distinct from the analogous, but 
not homologous, areolae, seen where the chondriform bone has invested the super- 
ficial cartilage-cells in the ossifying surfaces of the same fish. I may remark, that 
in this exatnple also the laminae of the parapophyses and neurapophyses are evidently 
continuous with those passing into the interior of the centrum. 
The only other fish which I propose to notice in the present memoir is the Salmon 
{Salmo Linn.), the bones of which present a singularly anomalous structure. In 
their external contours they do not appear to differ materially from those of the other 
osseous fishes, with the exception that the ichthyotomist finds them very difficult to 
dissect and disarticulate, owing to the circumstance that there exists in connection 
with them a large amount of cartilaginous tissue of a tough leathery consistency. 
On making a vertical section of a vertebra in the same direction as that of the 
Pike (fig. 41), we perceive that there is a similar arrangement of the osseous and carti- 
laginous segments. The osseous tissue is full of areolse of various forms, resembling 
those of the chondriform bones of Plagiostomes. They vary in shape, being some- 
times spherical and at others elongated and fusiform. Instead however of growing 
merely at their extremities, the osseous elements increase wherever they come into 
contact with the cartilage, the increase proceeding the most rapidly at the peripheral 
portions of the osseous segments or plates. Where the cartilage is in contact with 
the sides of these plates, it is converted into a very marked form of ^6ro-cartilage, 
the fibres being exceedingly distinct, running in radiating lines from the centre of the 
organism, and of course being parallel with the growing lateral surfaces of the plates 
or segments. This altered character of the cartilage is equally obvious where it in- 
vests the peripheries of the radiating plates, and in all cases the cartilage-cells are 
retained amongst its parallel fibres, though considerably altered in their form and 
appearance. At the internal angle of each cartilaginous segment and at the bases of 
the neurapophyses, there exists a small amount of chondriform bone of a character 
which more closely resembles that in the corresponding portions of the vertebra of 
the Pike. In the Salmon, its areolae are larger in proportion to the amount of the 
solid calcareous substance with which they are surrounded. 
All the other bones exhibit a similar process of development to those of the osseous 
