676 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
additions are made to two. We have seen this to be the case, not only with the 
osseous cylinder immediately surrounding the chorda, but also with the superior 
and infei ior ]amin(3e ; and there also appear to be indications of two lateral laminae, 
formed in the same way, immediately above the parapophyses. We shall shortly find 
that this is the first step in some important physiological changes. 
In the vertebrae of the large Carcharias vulgaris, Cuv., so common in osteological 
collections, the two terminal cones are united by four transverse bony plates ; the two 
lateral ones are very large, whilst the intermediate ones are much smaller. These 
plates, as Muller, Professor Owen and others have already pointed out, are sepa 
rated by deep, subcompressed, conical cavities, corresponding with the positions of 
the neurapophyses and parapophyses. Besides these large plates, there projects into 
the intervening cartilages four smaller ones ; consequently a transverse section of a 
dried vertebra made midway between the two terminal cones will present the ap- 
pearance of fig. 33, Plate XXXI. 
In this section, the four cavities (33 a) have been occupied by cartilage, whilst the 
entire shaded surface is composed of chondriform bone. It consists of an innu- 
merable series of minute areolse or cavities, which, when more highly magnified, 
present the appearance seen in fig. 34. Professor Owen describes the vertebrae of 
the Tope (Galeus communis), and most sharks possessing the nictitating eyelid, as 
having their external surface, as well as the terminal cones of the centrum, co- 
vered by a smooth osseous crust, except at the openings of the four cavities. In the 
present example, the bone is not merely a crust, but exists as four solid cones, the 
apices of which unite at the middle of the centrum, from whence also are given olF 
the small thin intervening plates, 33 c. The whole of this structure has been formed 
in the same way as the corresponding portions of the vertebrae of the Ray and Dog- 
fish. The areolm (34 a) are large, irregularly oval, and more or less compressed. 
The intervening calcareous portions (34 b) are small in proportion to the size of 
the areolar cavities, especially when compared with the relative development of the 
same textures in the Ray, where the earthy element, is so much more abundant. 
These calcareous septa in the Carcharias are rough and somewhat irregular in their 
distribution, with the exception of a few more definite rows which take their rise 
from the lateral margins of each cone, 33 d, and proceed towards their peripheries. 
These rows are probably the result of lateral additions made to the cones during their 
process of increment. 
A vertical section of the same vertebra, made at right angles to the last, along the 
dotted line, fig. 33 e e, reveals a somewhat difFererjt arrangement of the areolae. The 
contour of such a section, unmagnified, is seen in fig. 36. The central portion of 
each half (36 a) is occupied by areolse, like those, of which a magnified representation 
is given in fig. 34 ; but at each concave articular margin (36 b) there is an external 
layer, having a somewhat different structure. Fig. 35 represents a small portion 
removed from 36 a, and highly magnified. At the extreme margin (35 a) the areolse 
