10. All animals with horns are four-footed, unless there is any animal which metaphorically, and for the sake of a word, is said to have horns, as they say that the serpents in the neighbourhood of Thebes in Egypt have, though it is nothing more than an appendage, that is called a horn. The stag is the only animal that has solid horns, the horns of all other animals are hollow for a part of their length, and solid at the extremity; the hollow part is principally formed of skin, and round this is arranged the solid part, as in the horns of oxen. The stag is the only animal which casts its horns; they are reproduced; this takes place every year after the animal has attained the age of two years; other animals never lose their horns unless destroyed by violence. CHAPTER III. 1. The parts of the mammæ also, and the organs of generation, are different in man and in other animals. For some have the mammæ forward on or near the breast, and two mammæ with two nipples, as man and the elephant, as I said before, for the elephant has two mammæ near the arm-pits; in the female they are small, and do not bear any proportion to the size of the animal, so that they are scarcely visible in a side view; the males also have mammæ as well as the females, but they are exceedingly small. 2. The bear has four, other animals have two mammæ upon the thighs, and two nipples like sheep; others have four nipples, as the cow; some animals have not their nipples on the breast and thighs, but on the abdomen, as the dog and the hog, they have many nipples, but not all of the same size; other animals also have more than two, as the panther, which has four on the abdomen; the lioness has two on the abdomen, the camel has two mammæ and four nipples, like the cow. 3. Among animals with a solid hoof the males have no mammæ, except some horses which bear a resemblance to their dams. Some males have the penis external, as man, and the horse, and many others; some internal, as the dolphin. Of those animals in which it is external, some have it in front, as those which I have named; and some of these have both the penis and testicles loose, as in man; others have them close to the abdomen; some have them more, others less loose, for this part is not equally free in the boar and the horse.