Then followed a long development in house structure, differing materially from this earlier type. Horizontal masonry replaced the cruder attempts of house-wall construction; rectangular or squarish forms replaced the somewhat circular earlier type; and gradually the single-room structures were grouped into ever-enlarging units which assumed varying forms of arrangement as the development progressed. The art of pottery making improved concurrently with the more complex house structure. This later period represents the intermediate era of development from the crude Late Basket Maker dwellings to the remarkable structures of the "Cliff House Culture." During this period of transition new people penetrated the area. The Basket Makers throughout the course of their development were consistently a long-headed group. The appearance of an alien group is recorded through the finding of skeletons with broad or round skulls and a deformed occiput. These new people, the Pueblos, took over, changed, and adapted to their own needs the material culture of the earlier inhabitants. The Pueblos were not content with the crude buildings and earth lodges that sufficed as homes during the earlier periods. For their habitations they shaped stones into regular forms, sometimes ornamenting them with designs, and laid them in mud mortar, one on another. Their masonry has resisted the destructive forces of the elements for centuries. The arrangement of houses in a cliff dwelling the size of Cliff Palace is characteristic and is intimately associated with the distribution of the social divisions of its former inhabitants. The population was composed of a number of units, possibly clans, each of which had its more or less distinct social organization, as indicated in the arrangement of the rooms. The rooms occupied by a clan were not necessarily connected, and generally neighboring rooms were distinguished from one another by their uses. Thus, each clan had its men's room, which is called the "kiva." Each clan had also a number of rooms, which may be styled the living rooms, and other enclosures for granaries. The corn was ground into meal in another room containing the metate set in a stone bin or trough. Sometimes the rooms had fireplaces, although these were generally in the plazas or on the housetops. All these different rooms, taken together, constituted the houses that belonged to one clan. The conviction that each kiva denotes a distinct social unit, as a clan or a family, is supported by a general similarity in the masonry of the kiva walls and that of adjacent houses ascribed to the same clan. From the number of these rooms it would appear that there were at least 23 social units or clans in Cliff Palace.