678 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
are implanted the numerous minute dermal teeth. The calcareous chondriform 
prisms or rods exhibit some peculiarities. Their internal extremities exactly re- 
semble the whole structure of those existing in the jaws of Cestracion and Carcliarias; 
hut the outer half of each osseous plate is very different. In order to obtain so con- 
siderable a bulk of solid calcareous matter as is required to give the requisite 
strength to the rostrum of the Saw-fish, nature has slightly modified her plan, but 
without abandoning the type prevailing amongst the other Plagiostomes ; she has 
accomplished the object by adding largely to the length of each of these calcareous 
prisms ; and where several of them proceed inwards from different sides of an 
angular cartilage and meet near its centre, they are twisted about in a peculiar 
manner, so as to fit in between one another, and thus increase the solidity of the 
structure. 
It is evident that each of these prisms in the Saw-fish consists, when divided ver- 
tically, of two distinct portions, an internal and an external one. At its rounded in- 
ternal extremity, the areolae are usually arranged in a fan-like semicircle of radiating 
lines ; the same series is prolonged outwards like a hollow cone, which invests the 
inner half of the structure, and in which the cavities are arranged at right angles to 
its longer axis. The centre of this portion is occupied by a cone of a different struc- 
ture, containing cells differently arranged. The external half of the bone corresponds 
with the central cones. It is also full of cavities, but they are much less spherical, 
and in the transverse section form an irregular network. They appear to have been 
left by the imperfect coalescence of elongated botryoidal rods, of which this part of 
the structure consists. Each half of these prisms has obviously been formed in a 
different way from the other. Instead of the primary point of ossification having 
been at the outer extremity or base, as is certainly the case with those of the Ray, 
and apparently also of Carcliarias and Cestracion, in the present example I believe it 
to have been in the middle of the prism at the apex of the central cone ; and that 
whilst new additions have been made to the sides and inner extremity of each prism 
by calcification of the cartilage in the way already described, cognate additions have 
also been made to its base ; but these latter growtlis, which become continuous through- 
out the entire hone, and not subdivided into vertical prisms, have not been produced by 
the calcification of ordinary cartilage. They consist of a congeries of elongated bo- 
tryoidal rods separated by long irregular cavities, the appearance of which in the 
transverse section has been already referred to. Tliough these cavities owe their ex- 
istence to the impei'fect calcification of the soft tissues, they are different from the 
areoleeformed bythe investment of the globular cartilage- cells, at the opposite extremity 
of the prism. This continuous external portion has evidently been developed in a 
modified form of fibro-cartilage in which the cells have been less obvious. In the case 
of the Dog-fish, where we found that both external and internal additions were made 
to many of the ossifying points, I have already noticed that the cartilage in which the 
external growths were effected, assumed a much more fibrous aspect than that which 
