It has long been recognized that circular ruins in the Southwest differ from rectangular ruins, not only in shape but also in structural features, as relative position and character of kivas. The relation of the ceremonial chambers to the houses, no less than the external forms of the two, at first sight appear to separate them from the pure type.[30] They are more numerous and probably more ancient, as their relative abundance implies. [30] It is premature to declare that the kivas in circular ruins do not belong to the vaulted-roofed type simply from want of observation to that effect. In Penasco Blanco and other ruins of the Chaco Canyon group, as shown in ground plans, they appear to be embedded in secular rooms. Additional studies of the architectural features of circular pueblos are desirable. These circular ruins, in which group is included certain modifications where the curve of one side is replaced (generally on the south) by a straight wall or chord, have several concentric walls; again, they take the form of simple towers with one row of encircling compartments, or they may have a double wall with inclosed compartments. Many representations of semicircular ruins were found in the region here considered, some of which are of considerable size. The simplest form is well illustrated by the D-shaped building, Horseshoe House, in Hackberry Canyon, a ruin which will be considered later in this article. Other examples occur in the Yellow Jacket, and there are several, as Butte Ruin, Emerson, and Escalante Ruins, in the neighborhood of Dolores. In contrast to the village type consisting of a number of pueblos clustered together, but separated from each other, where the growth takes place mainly through the union of components, the circular ruin in enlarging its size apparently did so by the addition of new compartments peripherally or like additional rings in exogenous trees. Judging from their frequency, the center of distribution of the circular type lies somewhere in the San Juan culture area. This type does not occur in the Gila Valley or its tributaries, where we have an architectural zone denoting that a people somewhat different in culture from the Pueblos exists, but occurs throughout the “Central Zone,” so called, extending across New Mexico from Colorado as far south as Zuñi. Many additional observations remain to be made before we can adequately define the group known as the circular type and the extent of the area over which it is distributed. The following examples of this type have been studied by the author: WOOD CANYON RUINS