There is no clue to the purpose of these pits. Some have thought that they were made to hold the blood of sacrifice which was poured over the slab, and from some such idea may have arisen some of the legends of human victims which still cling round the dolmens. Others have opposed to this the fact that the pits sometimes occur on vertical walls or under the cover-slabs, and have preferred to see in them some totemistic signification or some expression of star-worship. It is possible that we have to deal with a complex and not a simple phenomenon, and that the pits were not all made to serve a single purpose. Those which cover some of the finest stones at Mnaidra and Hagiar Kim are certainly meant to be ornamental, though there may be in them a reminiscence of some religious tradition. In any case, it is worth while to remember that cup-markings also occur on natural rocks and boulders in Switzerland, Scandinavia, Great Britain (where there is a good example near Ilkley in Yorkshire), near Como in Italy, and in Germany, Russia, and India. Of the builders of the megalithic monuments themselves we cannot expect to know very much, especially while their origin remains veiled in obscurity. Yet there are a few facts which stand out clearly. We even know something about their appearance, for the skulls found in the megalithic tombs have in many cases been subjected to careful examination and measurement. Into the detail of these measurements we cannot enter here; suffice it to say that the most important of them are the maximum length of the skull from front to back and its maximum breadth, both measures, of course, being taken in a straight line with a pair of callipers, and not round the contour of the skull. If we now divide the maximum breadth by the maximum length and multiply the result by 100 we get what is known as the cephalic index of the skull. Thus if a skull has a length of 180 millimetres and a breadth of 135, its cephalic index is 135/180 X 100, i.e. 75. It is clear that in a roundish type of head the breadth will be greater in proportion to the length than in a narrow elliptical type. Thus in a broad head the cephalic index is high, while in a narrow head it is low. The former is called brachycephalic (short-headed), and the latter dolichocephalic (long-headed). This index is now accepted by most anthropologists as a useful criterion of race, though, of course, there are other characteristics which must often be taken into account, such as the height and breadth of the face, the cubic capacity of the skull and its general contour. At any rate, if we can show that the skulls of the megalithic tombs conform to a single type in respect of their index we shall have a presumption, though not a certainty, that they belong to a single race.