_Pygostyle._--This part of the skeletal system is variable in the species dealt with, not so much in size as in complexity. It reflects, of course, the character of the caudal muscles and their size, as well as the length of the rectrices and the corresponding force necessary to hold these feathers upright and in a useful position. Firm attachment is important even in flight, because the tail is used as a rudder, and in the Ptilogonatinae as a brake. The pygostyle is most modified in this subfamily. In lateral aspect, the pygostyles of the species of the Ptilogonatinae are similar. The crest of the bone is flattened dorsally, and has a broad anterior surface that is thin and bladelike. This is widest in _Ptilogonys caudatus_, and narrowest in _Phainoptila_, in which genus, however, the entire bone is of small size. The centrum is widest in _Ptilogonys caudatus_, and is progressively narrower in _P. cinereus_, _Phainopepla_, and _Phainoptila_. Greater width provides a larger area of attachment for the larger rectrices and also more area for insertion of the lateralis caudae muscle, the size of which varies more than that of the other caudal muscles in the different species of the Bombycillidae. [Illustration: Figs. 29-35. Pygostyles in posterior view of five genera of Bombycillidae. x 2. 29. _Phainoptila m. melanoxantha_, sex?, MNH no. 26493, 15 mi. SE Cartago, Costa Rica. 30. _Ptilogonys caudatus_, male, MNH no. 24492, 15 mi. SE Cartago, Costa Rica. 31. _Phainopepla nitens_, male, MNH no. 24754, Pima Co., Arizona. 32. _Ptilogonys cinereus_, female, Louisiana State University no. 297, Xilitla Region, San Luis Potosi, Mexico. 33. _Dulus dominicus_, female, USNM no. 292652, Don Don, Haiti. 34. _Bombycilla cedrorum_, male, MNH no. 15331, Bexar Co., Texas. 35. _Bombycilla garrula_, sex?, USNM no. 223895, Bozeman, Montana.] In proportionate size (see Table 7), the pygostyle of _Bombycilla_ is the smallest in the family. The dorsal spinous portion is acutely pointed instead of flattened as in the Ptilogonatinae. In _Dulus_, the spinous portion is extremely thin, and shows a decided curve dorsad from the centrum, and there is no flattened area anterior to the spinous portion as is seen in _Ptilogonys_. The centrum in cross section varies considerably. In _Bombycilla_ the walls are indented, with definite terminal knobs; both knobs and indentations are more pronounced in _B. garrula_ than in _cedrorum_, however. The spinous portion is enlarged in both species, and the rest of the neck region is constricted (Figs. 29-35).