Head-shape is really a far more complicated thing to arrive at for purposes of comparison than one might suppose. Since no part of the skull maintains a stable position in regard to the rest, there can be no fixed standard of measurement, but at most a judgment of likeness or unlikeness founded on an averaging of the total proportions. Thus it comes about that, in the last resort, the impression of a good expert is worth in these matters a great deal more than rows of figures. Moreover, rows of figures in their turn take a lot of understanding. Besides, they are not always easy to get. This is especially the case if you are measuring a live subject. Perhaps he is armed with a club, and may take amiss the use of an instrument that has to be poked into his ears, or what not. So, for one reason or another, we have often to put up with that very unsatisfactory single-figure description of the head-form which is known as the cranial index. You take the greatest length and greatest breadth of the skull, and write down the result obtained by dividing the former into the latter when multiplied by 100. Medium-headed people have an index of anything between 75 and 80. Below that figure men rank as long-headed, above it as round-headed. This test, however, as I have hinted, will not by itself carry us far. On the other hand, I believe that a good judge of head-form in all its aspects taken together will generally be able to make a pretty shrewd guess as to the people amongst whom the owner of a given skull is to be placed.