288 STRUCTURE AND CLASSIFICATION OF BIRDS 



In the skull the rostrum is articulated with the frontal 

 region by a well-marked hinge. The skull is holorhinal, 

 schizognathous, and there are no basipterygoid processes. 

 The vomer is expanded and bifid in front in a fashion that 

 recalls the segithognathous skull and that of certain of the 

 Charadriiformes (see below). 



In the young skull Parker figures basipterygoid pro- 

 cesses, not, however, articulating with the pterygoids ; they 

 appear to be not unlike those of Aptornis (see below). The 

 holorhinal nostrils are partly obliterated by an ossified ali- 

 hasal, as in so many picarian and passerine birds. 



The alimentary canal of Opisthocomus is remarkable in 

 more than one way. There is, in the first place, the enormous 

 crop, which has been most recently and most fully described 

 by GrADOW. This organ is very large, and rests upon the 

 furcula and the fore part of the sternum, for the abortion of 

 the anterior part of whose keel Gadow thinks the crop is by 

 its pressure responsible. The crop too is exceedingly muscu- 

 lar, and has numerous parallel folds in its interior, some of 

 which are continued into the oesophagus below. The gizzard 

 is much reduced in size. Probably the crop is not a mere 

 storehouse, but a compartment where at least trituration of 

 the food (chiefly leaves) takes place. 



The most remarkable feature about the intestine ' is 

 the long and coiled rectum, a feature which is also found 

 among the struthious birds and in the archaic Chauna. 

 The general arrangement of the coils of the small intestine is 

 intermediate between those of Pterocles and pigeons. There 

 is in the middle loop a faint trace of the spiral found in the 

 corresponding loop of the pigeon's gut. There are also like- 

 nesses to the form of gut in the cuckoos. The ccsca are 

 fairly developed. 



The peculiarities of the muscular system mainly concfern 

 the hind limb, and chiefly characterise the amhiens. The 

 muscle formula is complete, i.e. ABXY-I-. The ambiens, 

 however, is subject to variation. Garrod found that in all 



' P. Chalmebs Mitchell, ' A Contribution to the Anatomy of the Hoatzin 

 (Opisihocomiis cristatus),' P. Z. S. 1896, p. 618. 



