170 GENERAL ORNITHOLOGY part ii 



inferior coverts are the best distinguished from the general plumage, 

 the anus generally dividing off these "vent-feathers," as they are 

 sometimes called. It is to the bundle of under tail-coverts, behind 

 the vent, that the term crissum is most properly applied. Neither 

 set is ever entirely wanting ; but one or the other, particularly the 

 upper one, may be very short, as in a cormorant, or duck of the 

 genus Erismatura, exposing the quills almost to their bases. WhUe 

 the upper coverts are usually shorter and fewer than the under 

 ones, reaching less than half-way to the end of the tail, they some- 

 times take on extraordinary development and form the bird's 

 chiefest ornament. The gorgeous, iridescent, argus-ej^ed train of 

 the peacock consists of enormous tectrices, not rectrices ; the elegant 

 plumes of the paradise trogon, Pharomacnis mocinno, several times 

 longer than the bird itself, are likewise coverts. Occasionally, a 

 pair of coverts lengthens and stiffens, and then resembles true tail- 

 feathers, as in the ptarmigan (Lagopus). The crissal feathers are 

 more uniform in development ; they ordinarily form a compact, 

 definite bundle, as well shown in a duck, where they reach about to 

 the end of the tail. In some of the storks, they become plumes 

 of considerable pretensions known as marabous; and in the 

 wonderful humming-bird, Loddigesia mirabilis, the middle pair stiffens 

 to resemble rectrices and projects far beyond the true tail. The 



Reetriees, Rudders, or true tail-feathers, like the remiges or 

 rowers, are usually stiff, well-pronounced feathers, pennaceous to 



the very base of the vexilla, without 

 '^ after-shafts, as a rule, and with the 



outer web narrower than the other in 



most cases. They are always in 



pairs; that is, there is an equal 



number of feathers on the right and 



left half of the tail ; and their num- 



" ~l ber, consequently, is an even one. 



The exceptions to this rule are so few 

 and irregular, and then only among birds with the higher numbers 

 of rectrices, that such are probably to be regarded as mere anomalies, 

 from accidental arrest of a feather. They are imbricated over each 

 other in this wise : the central pair are highest, Ipng with both 

 their webs over the next feather on either side, the inner web of 

 one of these middle feathers indifferently underlying or overlying 

 that of the other ; all thus successively overlying the next outer 

 one so that they would form a pyramid were they thick instead of 

 being so flat. The arrangement is perceived at once in the accom- 

 panying diagram ; where it will be seen, also, that spreading the 

 tail is the divergence of a from b, while closing the tail is bringing 

 a and b together under c. The motion is effected by certain muscles 



