These are tumors formed largely of the peculiar fat which is found in bile and brain matter, and that crystallizes in flat oblong scales with a notch at one corner. The tumors are usually connected with the choroid plexus and developed beneath the pia mater, and may be of any size from a pea to a hen’s egg, or in exceptional cases a sheep’s kidney. As a basis there is a stroma of connective tissue permeated by blood vessels from the plexus. Groups of spherical or polygonal cells fill the interstices while fusiform cells are found in the stroma. There is a variable amount of phosphate or carbonate of lime which in oldstanding cases may give a cretaceous character to the mass. These constitute sandy tumors (psammomata). Cholesteatomata are especially common in old horses and are manifestly connected with congestion of the choroid plexus and exudation. In a recent case or in a case which has shown a recent cerebral hyperæmia, we may find a central mass of yellowish cholesterine, and surrounding this an abundant yellow gelatinoid exudation. This latter is rich in cholesterine which fails to dissolve along with the rest of the exudate on the occurrence of resolution, and is therefore laid up as the solid fatty material. For the same reason the fatty element is usually laid on in layers, one corresponding to each access of local hyperæmia and exudation. The great tendency to calcareous degeneration has been attributed to the abundance of phosphate of lime in the cerebral exudate. The _symptoms_ of these tumors are exceedingly uncertain. Many such tumors of considerable size have been found after death in animals in which no disease of the brain had been suspected during life. In these it is to be inferred that the accretions were slow, gradual, and without any serious congestion. In other cases the tumor is attended by paroxysms of vertigo, or indications of hyperæmia or meningitis, which will last for several days and gradually subside. It is reasonable to suppose that the tumors are largely the result of such recurrent attacks of encephalitis, and are no less the cause of their recurrence. The intervals of temporary recovery correspond to the subsidence of hyperæmia and the reabsorption of the liquid portion of the exudate. The manifestations during an access correspond directly to those met with in encephalitis. As in that affection there is usually an initial period of excitement and functional nervous disorder tending to more or less somnolence, stupor, paralysis or coma, with long intermissions of apparently good health. In other cases the stupor or paretic symptoms may persist up to the fatal issue. MELANOMA OF THE ENCEPHALON.