BIRDS 



413 



family possesses a bridge. The number of folds does not depend so much on the 

 species of bird as on its habits. Thus among the Palacognathn?, the active diurnal 

 ostriches, Struthio and Rhea, have 25 to 30 folds, the shy and crepuscular 

 cassowary, Casuarius, 4 large and 2 small folds (almost a cone), and the nocturnal 

 kiwi, Apteryx, none. 



In its general form the pecten assumes a number of variations 

 which, have been classified into 4 types by Casey Wood (1917) (Figs. 

 512 to 520 ; Plate XII) : 



(1) a stumpy structiu-e projecting only a short distance into the 

 vitreous, such as in the night heron, N^ yet icorax, and the secretary bird, 

 Serpentarius cristatus (Figs. 513-4) : 



(2) a curved structure sloping away from the visual axis ventrally, 



Figs. 512 to 520. — Types of Pecten in Birds 

 (The fovea when present is shown) (after Casey Wood). 



The night heron 

 Nycticorax 



The secretary bird 

 Serpentarius 



Fig. 512.— Tlie kiwi, 

 Apteryx. 



Fig. 513. — Tlie common 

 kestrel, Falco tinnuncuhts. 



Fig. 514. — The secretary 

 Vjird, Serpentarius. 



Fig. 515. — The herring- 

 gull, Larus argentatun. 



Fig. 516. — The wood- 

 pigeon, Columba 

 palutnbus. 



Fig. 517. — The American 

 ostrich, Rhea. 



Fig. 518.— The laughing 

 kingfisher, Dacelo yiyK.s. 



Fig. 519. — The cliimney 

 swallow, Hirumlo rusticu. 



Fig. 520.— The blue jay, 

 Cyanocitta. 



