Close scrutiny of the series of specimens at hand well representing the entire Rocky Mountain area reveals no variation in phylogenetic characters from the northernmost to the southernmost stations. All show in apparently equal degree the long tail and cinnamon tinge of sides and back, these features together constituting the grounds for separate subspecific recognition of a Rocky Mountain form. On the other hand, the Sierra Nevadan center, with its own recognizable race, [Page 507] of relatively short tail, proves to have two outlying divergent forms. These three forms are alike in their lack of any cinnamon tinge, this being replaced in two of them by a buffy tinge and in one form by leaden gray. The tail in one of the outlying forms is long, in the other short. The habitats concerned are, respectively, the desert mountains of the Inyo region of eastern California, and the coastal mountains of southern California. This differentiation within the Pacific district, particularly within the state of California, will be better understood in its geographic bearing by reference to the accompanying map (fig. 1). The behavior of the tail of _Penthestes gambeli_--long in the Rocky Mountain district, short in the Pacific district (see figs. 2, 3)--is paralleled in the _Penthestes atricapillus_ group of chickadees across the North American continent in about the latitude of the state of Washington. In the northern Rocky Mountains occurs the race _P. a. septentrionalis_, with long tail; in the Pacific Coast strip of Washington and Oregon occurs the race _P. a. occidentalis_, with, among other characters, relatively short tail. Other parallels are to be found in the genera _Psaltriparus_, _Thryomanes_ and _Pipilo_. [Illustration: Fig. 2. Tail of _Penthestes gambeli gambeli_ (no. 27784, Mus. Vert. Zool.; [Male], full-grown juv.; Sierra Ancha, Gila Co., Arizona; June 26, 1917). × 1.00.] [Illustration: Fig. 3. Tail of _Penthestes gambeli abbreviatus_ (no. 24059, Mus. Vert. Zool.; [Male], full-grown juv.; Yolla Bolly Mt., Tehama Co., California; August 5, 1913). × 1.00.] It is improbable that the fact of subspecific differentiation in _Penthestes gambeli_ has been altogether overlooked previous to the present time. A premonition of the geographic variation now formally pointed out is to be found in the early writings of Baird (1858, p. 394), who remarked that "a specimen, apparently of this species [_Parus montanus_], from Medicine Bow creek [Wyoming], (5643,) though marked female, is larger than those from California, as shown by the measurements." Also, some of the differences in proportions in the subspecies now recognized are shown in the table of measurements given by Ridgway (1904, p. 409). For a detailed description of the Mountain Chickadee as a _species_, the reader is referred to Ridgway (1904, p. 408). =Penthestes gambeli gambeli= (Ridgway) Rocky Mountain Chickadee