ALIMENTARY CANAL 21 



some added significance in view of the possible relationship 

 to modern birds of the cretaceous Ichth/yornis (cf. below). 

 The need for teeth seems to have disappeared with the 

 development of a horny bill, the replacement of -the one 

 structure by the other being, perhaps, comparable to the 

 replacement of functional teeth by horny plates in the 

 Ornithorhynchus. Functional teeth, however, existed in 

 the Jurassic Archmopteryx and Laopteryx (?), and in the 

 toothed birds of the cretaceous epoch, Hesperornis and 

 Ichthyornis. In the latter the teeth are in sockets, in the 

 former in grooves. In both the teeth are numerous, but 

 not, perhaps, extending on to the premaxillaries ; the teeth 

 show no specialisation in different regions ; they are of 

 dentine coated with enamel, and in Hesperornis the basal 

 portion of the roots consists of osteodentine. 



The Eocene bird (from the London clay) Odontopteryx 

 toliapicus has a strongly serrated upper jaw, a state of 

 affairs which is paralleled in the South American passerine 

 Phytotoma rara. In this latter bird, as Parker has pointed 

 out,' there is a ' row of clearly defined denticles, both along 

 the dentary and palatine ridges of the premaxillary.' He 

 suggests that in these birds and in the merganser, where 

 similar ' denticles ' occur, the bone of the jaw has grown 

 into arrested dental papillae. 



The oesophagus dilates in a few birds into a crop, which 

 is more highly specialised in Opisthoconius (q.v.) than in any 

 other form. When the crop is well marked it consists of a 

 spherical to oval dilatation of the oesophagus, which in 

 pigeons is divisible into a right and left half and an inter- 

 mediate unpaired portion. The gallinaceous birds, the 

 parrots, and among the • Limicolae the American genera 

 Thinocorys and Attagis, are provided with a crop. In other 

 birds a slight dilatation of the oesophagus, either permanent, 



papilleu bei Vogeln,' J.B. nat. Ges. Leipzig, 1882, p. 16 ; A. F. J. C. Mayeb, 

 ' Zahne im Obersohnabel bei Vogeln,' &a., Froriep's Notiz. xx. 1841, p. 69 ;. 

 Blanohard, ' Observations sur le Syst^me Dentaire ohez les Oiseaux,' Gomptes 

 Bend: 1. 1860, p. 540 ; M. Beadn, ' Die Entwioklung des Wellenpapageis,' Arb~ 

 Zool. Zoot. Inst. WUrzb. v. 1879. 



' In his memoir upon aegitbognathous birds. 



