184 STRUCTURE AND CLASSIFICATION OF BIRDS 



Twelve rectrices is the rule for this family, but the outer 

 ones are sometimes feeble, and I found only ten in Tig a 

 Shorei. The aftershaft is present. The pterylosis varies 

 somewhat. But NITZSCH has pointed out that this ' peculiarity 

 . . . which occurs almost universally among them, is the 

 presence of a small inner humeral tract running along upon 

 the most elevated points of the shoulder parallel to the 

 very broad main tract.' That this second humeral tract also 

 occurs among parrots may be a matter worthy of considera- 

 tion. It appears, at any rate, to distinguish the woodpeckers 

 from other picarian birds. 



In Picus viridis the narrow dorsal tract passes down the 

 middle of the neck and ends abruptly at the end of the 

 scapulae or a little before. Behind this are two oval wide 

 patches, which correspond to the foot of the Y in other 

 birds, which have a dorsal median apterion. There is a 

 break between these patches and the posterior end of the 

 dorsal tract, which runs to the base of the oil gland undivided. 

 On either side of this and of the oil gland is a fainter and 

 narrower tract. In Tig a Shorei there is no break between 

 the several regions of the dorsal tract ; the anterior part 

 narrows and forks into two branches, consisting of but one 

 row of feathers, which immediately after dilate into the wide 

 interscapular tracts ; from the lower angle of each of these 

 a single row of feathers joins the median posterior part of 

 tract. The diamond-shaped spinal aptsrion is thus com- 

 pletely enclosed within the dorsal tract. 



Sphyrapicus nuchalis ! has a solid spinal tract dilating 

 between the shoulders into a rhombpidal but still solid area, 

 as SHUFELDT says, like a passerine. 



In Centurus striatus the arrangement is more like that 

 of Picus viridis, but slight scattered feathering unites the 

 middle and posterior portions of the dorsal tract. 



The ventral tracts of Picus divide early in the neck and 

 at the commencement of the pectoral region ; each gives off a 

 stronger outer branch. In Tig a Shorei the separation between 

 these branches is not nearly so marked as is shown in 



1 'Observations on the Pterylosis of Certain PicidaV Auk, ISHS, p. 212. 



