Pico-Passerine Group of Birds. 37 



B. Picine Scansores, without caeca and with a tufted oil- 

 gland. 



Picida: vomer slender, pointed, split; maxillo-palatines free. 



Indicatoridae : vomer bifid ; maxillo-palatines free ; spinal 

 feather-tract surrounding a post-scapular naked space, but 

 otherwise continuous. 



Capitonidce : vomer bifid ; maxillo-palatines sometimes free, 

 sometimes coalesced ; spinal feather-tract interrupted by the 

 absence of the anterior postscapular fork. 



Rhamphastidce : vomer truncated ; maxillo-palatines co- 

 alesced. 



+ Heterodactyli. 



The Trogons are very curious birds. They agree with the 

 Scausores and the Pseudo-Scansores in having two toes in 

 front and two behind ; but they differ from both those groups 

 (and from all other birds) in having the second digit (the 

 inner front toe of anisodactyle birds) reversed. In a hete- 

 rodactyle bird the hallux is the outer of the two hind toes, 

 whereas in a zygodactyle bird the hallux is the inner one. 

 This perfectly unique arrangement of the toes is correlated 

 with a perfectly unique arrangement of the deep plantar 

 tendons. The front plantar leads to the two front toes, and 

 the hind plantar to the two hind toes. This arrangement 

 (apparently so simple and natural) is in strong contrast with 

 those of the two zygodactyle groups. In the Scausores the 

 front plantar leads to the third digit only, whilst in the 

 Pseudo-Scansores the hind plantar leads to the first digit 

 (the hallux) only. 



In other respects the Trogons are also remarkable. They 

 combine the cranial characters of Caprimulgus with the 

 pterylosis of Motacilla, and the thigh-muscles and sternum 

 of Alcedo. They are schizognathous and holorhinal ; and 

 they are the only birds in the Order of Pico-Passeres which 

 permanently retain their basipterygoid processes. 



The Heterodactyli consist of one family only, the Tro- 

 gonidse. 



