In nearly all mammals a further change takes place in the position of the anterior limb. The radius and ulna have hitherto been parallel to one another, but now the lower end of the radius, carrying with it the manus, comes to be rotated forwards round the ulna, so that the manus, as well as the pes, comes to be forwardly-directed, and its pre-axial surface faces inwards.

In the majority of mammals the radius and ulna are permanently fixed in this, which is known as the prone position, but in man and some other mammals the manus can be pronated or turned into this position at will. When the radius and ulna are parallel throughout their whole length the manus is said to be in the supine position.

The extensor side of a limb is that to which the muscles which straighten it are attached, the flexor side is that to which the muscles which bend it are attached.

CHAPTER II.
CLASSIFICATION.

The following classification includes only the forms mentioned in the succeeding pages. The relative value of some of the terms employed in classification is not identical throughout the book. This remark applies specially to the term group, which is a convenient one, owing to its not having such a hard and fast zoological meaning as has the term family, for instance. The term group is applied in this book to divisions of the animal kingdom of very different classificatory importance.

PHYLUM CHORDATA.

SUBPHYLUM A. HEMICHORDATA.