670 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
laginous skeleton is covered with “an osseous crust, which is composed of a great 
number of small plates or rod-like portions of bone united in the manner of mosaic 
work*.” Mr. Millar has also described the structure somewhat more in detail, as 
forming the osseous covering of the cartilaginous skulls of the Sharks and Rays'l' ; 
but neither of these authors has noticed the minute structure of these plates, or re- 
cognized their true physiological relationship to other forms of ichthyal and mamma- 
lian bone. Professor Owen, in his admirable lectures, advances somewhat farther; he 
says, “in some species (of fishes) ossification commences at the periphery of the 
animal mould or basis, and is always limited to a thin outer crust of the bone, the rest 
remaining cartilaginous or gelatinous. In some of the higher cartilaginous fishes, for 
example, an osseous crust is formed upon the periphery of certain cartilages in the 
form of prisms, which contain oval calcigerous cells, but without conspicuous radiated 
tubes:|:.” Still even the accomplished author of the above passage does not appear to 
have observed the difference between these “ calcigerous cells ” and the canaliculated 
lacunse of more highly organized bone; they are however perfectly distinct. 
Fig. 28 exhibits a vertical section of the semicartilaginous column of the Raia cla- 
vata, or common Thornback. The section has been made transversely, crossing one 
of the vertebree midway between its two concave extremities, the neural spine being 
removed ; the neural canal (28 a) is surrounded by a layer of small calcareous plates, 
28 Zj, and a corresponding series (28 c) covers the entire exterior of the structure, 
including not only the centrum and all the vertebral apophyses, but existing also 
along the lines of demarcation (28 d) separating the ossa intercalaria, which, in this 
group of fishes, often enter into the composition of the neural ring. 
Fig. 29 represents a horizontal section of a portion of this osseous crust, taken 
from the lateral surface of the neural spine, and shows the way in which the small 
calcareous plates are fitted together. Each plate thus examined exhibits six or eight 
short, broad processes, 29 a, which meet corresponding ones projecting from the 
contiguous plates, 29 h. Sometimes these processes are not all arranged on one 
horizontal plane, but are on different levels, as at 29 c, where the upper pair is want- 
ing, their deficiency being supplied by others deeper down. The uniform direction 
of these processes, modifying as they do that of the internal cells shortly to be 
noticed, give a stellate appearance to the surface of each plate ; and their partial 
apposition through the projecting processes causes the existence of numerous small 
circular and oval interspaces, 29 c and d, which are occupied by a modified form of 
cartilage. 
Fig. 30 represents one of these ossified plates still more highly magnified. The por- 
tions 30 a, a' are the projecting processes w’hich have met corresponding ones from 
contiguous plates, whilst the intervening indentations (30 h) have contributed their 
* Muller’s Elements of Physiology; translated by W. Baly, M.D., 2nd edit., 1840, p. 393. 
t Footprints of the Creator, p. 41, fig. 7. 
I Lectures on the Comparative Anatomy and Physiology of the Vertebrate Animals, Part I. p. 33. 
