36.Phainoptila m. melanoxantha, sex?, MNH no. 26493, 15 mi. SE Cartago, Costa Rica.
37.Ptilogonys caudatus, male, MNH no. 24492, 15 mi. SE Cartago, Costa Rica.
38.Phainoptila nitens, male, MNH no. 24754, Pima Co., Arizona.
39.Ptilogonys cinereus, female, Louisiana State University no. 297, Xilitla Region, San Luís Potosi, Mexico.
40.Dulus dominicus, female, USNM no. 292652, Don Don, Haiti.
41.Bombycilla cedrorum, male, MNH no. 15331, Bexar Co., Texas.
42.Bombycilla garrula, sex?, USNM no. 223895, Bozeman, Montana.

In specialization for muscle attachment, the centra of the pygostyles of the Ptilogonatinae have more area for muscle attachment than do the centra in the Bombycillinae and Dulinae; the centrum is wide, the spinous portion is long, and the bone is flattened anteriorly. The most generalized pygostyle is in Phainoptila, and that of Dulus differs only slightly. In Bombycilla the pygostyle is proportionately small, but is complex in shape; there is seemingly not the need for greatly expanded areas since the caudal muscles are less specialized in this genus.

Sternum.—The sternum in Bombycillids is typically passerine in general shape and in having a long and deep carina or sternal crest. The caudal process of the bone is broad, with the terminal ends flattened, forming dorsally a graceful V-shaped outline, whereas the outline of the posterior end of the sternum is broad and convex.

In lateral aspect, the carina is deeper in Bombycilla than in other genera of the family, and is deepest in B. garrula. In this species, the manubrium is more extended and comparatively larger than in the other species of the family. The anterior edge of the keel forms the sharpest angle in B. cedrorum. In Dulus, the keel is moderately deep, the manubrium short, and there is a distinct indented curve between the manubrium and the anterior angle of the keel.

In ventral aspect the lateral processes of the sternum tend to flare outwards in adult Ptilogonatines on almost the same plane as the rest of the bone, whereas in Bombycilla and Dulus the same process is closer to the body of the sternum. In Bombycilla the xiphoid process is more dorsal in position than in other species in the family, and in Dulus an upward curve is very noticeable. The process in these two genera is narrower than in the Ptilogonatinae, and lacks the heavy distal terminal enlargement which is apparent in Ptilogonys.

Relative Lengths of Bones.—In instances where the animals being compared are obviously different in over-all size, it is useful to express the size of a given part in relation to some other part of the same individual organism if the aim is to obtain clues as to differences in functions of the parts being compared. Differences in actual lengths of corresponding bones in two kinds of animals often, of course, reflect only the difference in over-all size of the animals. Consequently, the relative size of the part is expressed as a percentage in this paper. In computing a percentage it is well, of course, to select some relatively stable part of the animal to use as a denominator in the mathematical expression that yields the percentage. The thoracic region of the vertebral column is thought to be such a part. For example, the length of the humerus divided by the length of the thoracic region yields, in Phainopepla and Ptilogonys, respective percentages of .84 and .85. These are roughly the same, whereas the actual lengths of the humeri are 2.21 and 2.39 cm.

Table 4. Lengths of Leg Bones in cm.

SpeciesFemurTibiotarsusTarsometatarsus
Ptilogonys caudatus2.043.101.94
Ptilogonys cinereus1.892.901.77
Phainopepla nitens1.762.781.72
Phainoptila melanoxantha2.433.772.58
Dulus dominicus2.093.342.09
Bombycilla garrula2.323.461.99
Bombycilla cedrorum1.922.951.64