542 PHILOSOPHICAL TRANSACTIONS. [ANNO 1784. 



their being transparent, unless viewed with a deep magnifier, when a few opaque 

 lines may be perceived, which point out the cellular part of the tooth through 

 which the blood vessels ramify, which are destined for its growth and nourish- 

 ment. Those in the anterior parts of the jaws are the longest, whence they gra- 

 dually diminish in length as they approach the angles of the mouth. From this 

 description of the teeth of the chsetodon nigricans, this fish seems to be mis- 

 placed in the Systema Naturae of Linneus; since one generic distinction of the 

 chaetodontes is to have numerous, slender, and flexible teeth ; whereas the teeth 

 of the chaetodon nigricans are few in number, placed in one row, and of a crys- 

 talline hardness. 



Oj the teeili of cartilaginous fishes. — When Steno examined the teeth of the 

 shark, he was surprized to find a great number of them placed on the inside of 

 each jaw, lying close to the bone, and many of them buried in a loose spongy 

 flesh; concluding that these internal teeth could be of little or no use to the ani- 

 mal. Mr. Herissant afterwards showed the use of these internal or posterior 

 teeth, by proving, that as the anterior teeth of each row are broken off, drop 

 out, or wear away, the posterior ones come forward to supply their places. But 

 though it be certain that the anterior teeth, when lost, are replaced by the pos- 

 terior ones, none of the naturalists have attempted to ascertain how often this 

 circumstance happens. Whether the renovation be perpetual during life; or 

 whether that operation be suspended after a limited number of teeth have been 

 supplied. From a singular circumstance, which Mr. A. once met with, he is 

 inclined to think the former is the fact; or, that in cartilaginous fishes, such as 

 sharks, rays, &c. there is a perpetual renovation of the teeth. Being engaged 

 in dissecting the jaws of a very large shark, he found a portion of that sharp 

 bearded bone found in the tail of the fire-flaire, or sting-ray, driven quite through 

 the lower jaw among the posterior teeth, and fixed almost immoveably. How 

 this happened must be obvious to every one. 



The posterior teeth of cartilaginous fishes are always found in a soft, membra- 

 nous state, and but imperfectly formed; yet they have the whiteness of teeth 

 from a small quantity of calcareous earth already deposited within their substance. 

 Their hardness and perfect form is acquired as they advance towards the anterior 

 parts of the jaws. Of the 3 angles in each tooth of the shark, one is placed to- 

 wards the right, another towards the left, and the other, which is in the middle, 

 and the most acute angle, is directed inwardly towards the tongue or fauces. 

 They are placed then in such a maimer as that the angles of the teeth on the 

 left side in one row, approach the angles of the teeth on the right side in the 

 next row. As it is certain, that the anterior teeth were formerly posterior ones, 

 and as the teeth in each row were all deficient in one angle, it follows, that they 

 must have been formed posterior to the insertion of this extraneous body. Again, 



