SEC. Ill EXTERIOR PARTS OF BIRDS 173 



have the fewest, as twelve, sometimes fourteen, rarely sixteen ; 

 those with short soft tails have the most, as sixteen to twenty -four. 

 Among the penguins there are thirty-two or more. The Archceo- 

 pteryx appears to have had forty, — a pair to each free caudal vertebra; 

 and this may be considered the prototypic relation between the 

 bones and feathers of the tail. The 



Typical Shape of the Tail, as a whole, is the fan. The modi- 

 fications of form, however, which are greater and more varied than 

 those of the wing, are susceptible of better definition, and many of 

 them have received special names. Taking the simplest case, where 

 the rectrices are all of the same length, we have what is called the 

 even, square, or truncate tail. The other forms depart from this 

 mainly by shortening or lengthening of certain feathers. A tail 

 nearly or quite even may have the two central feathers long-exserted, 

 as seen in the jaegers (Stercorarius), and tropic-birds (Phaetkon). 

 The most frequent departure from the even shape results from 

 gradual shortening of successive rectrices from the middle to the 

 outer ones. This is called, in general, gradation or graduation (Lat. 

 gradus, a step) ; such shortening may be to any degree. More 

 precisely, graduation means shortening of each successive feather to 

 the same extent, — say, each half an inch shorter than the next ; 

 but such exactitude is not often expressed. When the feathers 

 shorten by more and more, we have the true rounded tail, probably 

 the commonest form among birds ; thus, the gradation between 

 the middle and next pair may be just appreciable, and then increase 

 regularly to an inch between the next and the lateral feather. The 

 opposite gradation, by less and less shortening, gives the wedge- 

 shaped or cuTieate (Lat. cuneus, a wedge) tail ; it is well shown by 

 the magpie {Pica) in which, as in many other birds, the middle 

 feathers would be called long-exserted were the rest all as short as 

 the outer one is. A cuneate tail, especially if the feathers be 

 narrow and lanceolate, is also called acute, or pointed, as in the 

 sprig-tailed duck (Dafila) or sharp-tailed grouse (Pediacetes). The 

 generic opposite of the gradated is the forked tail; in which the 

 lateral feathers successively increase in length from the middle to 

 the outermost pair. The least appreciable forking is ca,lled emar- 

 ginatim, and a tail thus shaped is said to be emarginate ; when it is 

 better marked, as, for instance, an inch of forking in a tail six 

 inches long, the tail is truly forked or furcate (Lat. furca, a fork). 

 But the degrees of furcation, like those of gradation, are so insen- 

 sibly varied, that qualified expressions are usual ; as, " slightly 

 forked," " deeply forked." Deep furcation is usually accompanied 

 by more or less narrowing or filamentous elongation of the lateral 

 pair of rectrices, as in the barn swallows {Hirundo) and most of the 

 sea-swallows {Sterna). An advisable term to express such an ex- 



