PILRVEOSTS. 745 
a fallacy to look on this feature as proof of an archaic condition 
in them, since fully-developed embryos of both Struthio and Apteryx 
have well-defined pteryle. If treated skilfully, Pterylosis is of prime 
taxonomic importance in Ornithology, though more in the investi- 
gation of small than of large groups. Unfortunately it can seldom 
be described in a few words, and hence it is chiefly or only those 
among its characters which can be expressed in terse and trim 
formule that appeal most 
to the mechanical con- 
structor of classifications.1 
The principal pterylex 
or feathered tracts are as 
follows :— 
(1) Spinal tract (pt. 
spinalis), extending along 
the vertebral column from | 
neck to tail, bordered by |, 
the lateral, cervical and 
trunk apteria or featherless 
spaces. This tract is one 
of the most variable, its 
modifications, of which 
Nitzsch enumerated 17, 
being practically count- 
less. It is rarely of the 
same width throughout, 
and is most frequently 
dilated on the back or 
between the shoulders, 
with or without a featherless space in the midst, the position and 
size of which varies much. In Pelecanus, Fregata, Phaethon and Ardea 
the space is narrow, and extends from the neck to the tail, in others 
as Podicipes, Cuculi, Cypselus, Coracias and Opisthocomus it is re- 
stricted to the back, in Sula to the interscapular region, in Colymbus 
to the neck. In some birds this apteriwn, whether interscapular, 
dorsal or lumbar, is rhomboidal, and it may become so large as to 
interrupt the spinal pferyla, which may end in an interscapular fork 
and begin again with a sacral bifurcation, or as a single streak ; but 
there is no apterium in the spinal pteryla of the following :—Latite, 
Sphenisci, Phalacrocorax, Plotus, Palamedea, Tinami, Galline (pt.), 
Sieey ON 
i 
. 
. 3 
CHARADRIUS PLUVIALIS. Ventral and dorsal aspect. 
(After Nitzsch.) 
1 Even this has taken place within comparatively few years, for Nitzsch’s 
great work on the subject Plerylographie (Halle: 1840, 4to), which after his death 
was edited by Burmeister, excited but little and mostly unfavourable notice for 
nearly a quarter of a century after its publication. An English translation by 
the late Mr. Dallas was brought out in folio by Mr. Sclater for the Ray Society 
in 1867, 
