SEC. Ill EXTERIOR PARTS OF BIRDS 185 



before (Fig. 37), or behind, or both, as the case may be. The term 

 is equally applicable to the acropodium, but is not so often used 

 because scutellation of the upper sides of the toes is so universal as 

 to be taken for granted unless the contrary condition is expressly 

 said. The most notorious case of the Osdne podotheca (Figs. 36, 37), 

 characterising that great group of birds, is given in the next 



Plates, or reticulations (L&t. reticulum, a web; Fig. 38, a) result 

 from the cutting up of the envelope in various ways by cross lines. 

 Plates are of various shapes and sizes, and grade usually into true 

 scutella, from which, however, they are generally distinguished by 

 being smaller, or of irregular contour, or not in definite rows, or 

 lacking the appearance of imbrication; but there is no positive 

 distinction. They are oftenest hexagonal (six-sided), a form best 

 adapted to close packing, as shown very perfectly in the cells of 

 the honey-bee's comb ; but they raay have fewer sides, or be poly- 

 gonal (many-sided) or even circular ; when crowded in one direction 

 and loosened in another the shape tends to be oval or even linear. 

 A leg so furnished is said to be reticulate : the reticulation may be 

 entire, or be associated with scutellation, as often happens (Fig. 38, 6). 

 A particular case of reticulation is called granulation (Lat. granum, 

 a grain), when the plates become elevated into little tubercles, 

 roughened or not. Such a leg is said to be granular, granulated or 

 rugose: it is well shown by parrots, and the osprey (Pandion). 

 When the harder sorts of scales or plates are roughened without 

 obvious elevation, the leg is said to be scabrous or scarious (Lat. 

 scabrum, a scab). But scabrous' is also said of the under surfaces of 

 the toes, when these develop special pads, or wart-like bulbs (called 

 tylari) ; as is well shown in many hawks. The softer sorts of legs, 

 and especially the webs of swimming birds, are often marked cross- 

 wise or cancellated with a latticework of lines, these, however, not 

 being strong enough to produce plates ; it is more like the lines 

 seen on our palms and finger-tips. The plates of a part of the leg 

 occasionally develop into actual serrations / as witnessed along the 

 hinder edge of a grebe's tarsus. When an unfeathered tarsus shows 

 no divisions of the podotheca in front (along the acrotarsium), or 

 only two or three scales close by the toes, it is said to be booted or 

 greaved ; and such a podotheca is holothecal {Gr. oXoi, holos, whole, 

 entire, and 0-qKr] ; Fig. 36). The generic opposite is schizothecal 

 (Gr. crxt'^M, I cleave), whether by scutellation, or reticulation, or in 

 any other way the integument may be cut up. A booted or holo- 

 thecal tarsus chiefly occurs in the higher Oscines, and is supposed 

 by ornithologists to indicate the highest type of bird structure. It 

 is, however, found in a few water birds, as Wilson's stormy petrel 

 and other species of Oceanites. It is not a common modification. 



