bones we see cartilage and osseous structure connected, without any such interposition; and no one expects such a matrix for the vitreous table of the cranial parietes. The crusta petrosa may have its uses apart from any conjectural function of secretion; and, in further proof that it does not secrete, it can, in the young subject, before the tooth has emerged from its primary cavity, be separated with the greatest facility from the enamel. It serves to protect the enamel, but it likewise answers the end of keeping the tooth firm in the jaw: around the neck of the molars it becomes thickened to a great extent; and at the roots, especially of old teeth, it exhibits considerable substance. The horse's molars are continually being wrenched from side to side while the food is being ground, and unless they were very firmly mortised, the vessels which nurture them would be continually lacerated: this the crusta petrosa, by entirely filling the alveolar cavity, prevents; and as the molars wear down with age, the thickening of the membrane enables the jaw to retain with security the latest portion of the tooth. I have specimens taken from very aged subjects, where the fang having been worn away, the molar consists merely of the roots of teeth embedded in a mass of crusta petrosa. 16. _The various parts and peculiarities of form_, now require consideration. If the table of an incisor tooth be observed, a cavity will be seen in the centre of it: this cavity, called the infundibulum, is of variable depth. In the temporary or milk teeth, it extends only half way down the crown; in the permanent or horse incisors, it is from three quarters of an inch to an inch and a quarter deep, being of greater depth in the teeth of the upper than in those of the lower jaw, and not of the same magnitude in all horses. The infundibula are indentations in the body of the ivory, and they possess two coverings, an external one of crusta petrosa, and internal investment of enamel: the enamel does not materially vary in thickness upon this part, but the crusta petrosa of the infundibula is, in different subjects, of very unequal extent. In some animals it becomes very thick, and in such, the infundibula seem soon to be obliterated, owing to the crusta petrosa filling up the cavity. A good specimen of the early obliteration of the cavity, by the thickness of its investing membrane, is shown by the teeth preserved with the jaw of the horse Leander, which is now in the possession of Mr. W. Field. That animal, though but four years old, had lost the "marks" in the central incisors. A white line, however, shows that the enamel dips deep into the substance of the ivory, and indicates what would have been the form of the infundibula, had not the crusta petrosa filled up the cavities. The infundibula have received special attention from horsemen, and have been thought to indicate correctly the age of the horse; their lining membrane speedily becomes blackened by