PTERYLOSIS 



745 



a fallacy to look on this feature as proof of an archaic condition 

 in them, since fully-developed embryos of both Struthio and Ai)teryx 

 have well-defined pterylx. 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 

 formulae that appeal most 

 to the mechanical con- 

 structor of classifications.^ 



The principal pterylse 

 or feathered tracts are as 

 follows : — 



(1) Spinal tract {pf. 

 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 Avhich 

 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 Felecanus, Fregata, Phaethon and Ardea 

 the space is narrow, and extends from the neck to the tail, in others 

 as Fodicipes, Cuculi, Cypselus, Conicias and Opisthocoimis it is re- 

 stricted to the back, in Sida to the interscapular region, in Colymbus 

 to the neck. In some birds this apterium, whether interscapular, 

 dorsal or lumbar, is rhomboidal, and it may become so large as to 

 interrupt the spinal p)teryla, 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 : — Batitx, 

 Sphenisci, Fhalacrocorax, Flotus, Falaniedea, Tinami, Gallinse (pt.), 



^ Even this has taken place within comparatively few years, for Nitzsch's 

 great work on the subject Pterylogrcqihie (Halle : 1840, 4to), which after his death 

 was edited by Burnneister, excited bnt 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. Sclatcr for the Ray Society 

 in 1867. 



Chakadrius PLtTviALis. Ventral antl dorsal aspect. 

 (After Nitzscli.) 



