No. 1. Wickyup Canyon, Ruin 1 and Ruin 2, “Boulder Castle.” No. 2. Two towers in Ruin Canyon: 1ᵃ, near the mouth; 1ᵇ, Towers on or near forks, No. 1 [Hovenweep Pueblo], No. 2 [Hovenweep Castle.] No. 3. [Square Tower.] No. 4. [Oval Tower.] No. 5. [Tower.] No. 6. [6.] No. 7. [Boulder Cliff-house.] No. 8. Twin Towers. No. 9. [9.] No. 10. [Unit type House.] No. 11. Gibraltar House and ruin. [Stronghold House.] No. 12. [12.] The pueblos and cave dwellings of the “Pivotal group” (those on or near the promontory at the junction of the McElmo and Yellow Jacket Canyons) were also studied by the authors. Almost the whole article by Morley and Kidder, which the editor announces will be completed in a future number of “El Palacio,” is devoted to descriptions of buildings[10] in Ruin and Road (Wickyup) Canyons and the ruins of the “Pivotal group” at the base of a promontory between the junction of the Yellow Jacket and McElmo. [9] The Archaeology of McElmo Canyon, Colorado. El Palacio, vol. iv, no. 4, Santa Fe, 1917. [10] The dimensions of buildings and towers given in this article are welcome additions to our knowledge, but from lack of ground plans one can not fully determine the arrangement of rooms designated in individual ruins. CLASSIFICATION In the classification by Morley and Kidder and the majority of writers, sites rather than structural features are adopted as a basis although all recognized that large cliff-dwellings like Cliff Palace are practically pueblos built in caves. In the following classification more attention is directed to differences in structure than to situation, notwithstanding the latter is convenient for descriptive purposes. 1. Villages or clusters of houses, each having the form of the pure pueblo type. The essential feature of the pure type is a compact pueblo, containing one or more unit types, circular kivas of characteristic form, surrounded by rectangular rooms. These units, single or consolidated, may be grouped in clusters, as Mitchell Spring or Aztec Spring Ruins; the clusters may be fused into a large building, as at Aztec or in the community buildings on Chaco Canyon. 2. Cliff-houses. These morphologically belong to the same pure type as pueblos; their sites in natural caves are insufficient to separate them from open-sky buildings. 3. Towers and great houses. These buildings occur united to cliff-dwellings or pueblos, but more often they are isolated. 4. Rooms with walls made of megaliths or small stone slabs set on edge.