The key to the culture of the people from which the cliff-dweller culture was derived is probably the kiva, which furnishes also a good basis for the classification of the Pueblos and cliff-dwellers into subordinate groups. Architecturally the kiva reached its highest development in the Mesa Verde region, where it is a circular subterranean room with pilasters and banquettes, ventilators and deflectors, fireplaces and ceremonial openings, the features of which have been described elsewhere. As we follow the San Juan down to its junction with the Colorado we find a gradual simplification of the circular type of kiva by the elimination of pilasters, ventilators, and other features, the round kiva being here represented by rooms in which almost the only architectural feature remaining is the large banquette. The question naturally arising in this connection is, whether the circular kiva in the eastern region is a development of that simpler form existing in the western or whether the latter is a degenerate form of the eastern. In other words, does the evidence show that this particular modification spread from the east down the San Juan or from the west up the river to the east? In this connection it may be urged that originally the form of circular kiva lacking pilasters extended along the entire course of the San Juan and that the kivas of the Mesa Verde became highly specialized forms in which pilasters were developed, while those lower down the river remained the same. We can not definitely answer either of these questions, but taken with other evidence it would seem that the circular form of kiva originated in the eastern section and gradually extended westward. [Illustration: BUREAU OF AMERICAN ETHNOLOGY BULLETIN 50 PLATE 19 CLIFF-DWELLERS CRADLE—FRONT Dimensions: length, 22 inches; breadth, 9 inches; diameter, 6 inches.] [Illustration: BUREAU OF AMERICAN ETHNOLOGY BULLETIN 50 PLATE 20 CLIFF-DWELLERS CRADLE—REAR] [Illustration: BUREAU OF AMERICAN ETHNOLOGY BULLETIN 50 PLATE 21 CLIFF-DWELLERS CRADLE—SIDE] The modern Hopi rectangular form of ceremonial room situated underground seems in some instances to have derived certain features from the circular subterranean kiva.