Gallino-Gralline Group of Birds. 417 



diate between the two in having the posterior outline of the 

 lateral section straight^ and differs from both in having the 

 anterior outline of the vertical section straight, whilst in the 

 anterior outline of the lateral section it agrees with the 

 Pheasant and not with the Penguin. In order to make the 

 term opisthoccelous cover the articulation of the dorsal ver- 

 tebrae of the Impennes and the Gavio-Limicolae, and exclude 

 that of the other suborders, it must be artificially defined as 

 follows: — Opisthoccelous vertebrae are, posteriorly, either 

 straight or concave (not convex) when seen in lateral section. 

 As thus understood (its taxonomic importance reduced to a 

 minimum), opisthocoelism becomes a useful character for 

 diagnostic purposes. Two facts in connexion with this cha- 

 racter are important : typically heterocoelous dorsal vertebrae 

 may or may not be ankylosed together, and may or may 

 not have deep lateral depressions ; opisthoccelous dorsal 

 vertebrae are never ankylosed together, and always have deep 

 lateral depressions. 



Ankylosis of the Dorsal Veriebrce. 

 The sacrum is composed of ankylosed vertebrae, in front of 

 which is a free dorsal vertebra. Four completely ankylosed 

 dorsal vertebrae precede this free dorsal vertebra in the Co- 

 lumbae, the Pterocletes, the Crypturi, the Gallinae, and the 

 Podicipesj but, as usual, the character breaks down in one 

 group. In the Grallae, Rhinocheius and Opislhocomus are 

 typically Galline in this respect ; Psophia and some species 

 of Grus and Otis are partially so ; whilst in most species 

 all the vert€bra3 in front of the ankylosed sacral vertebrae 

 are free. 



Ventral Processes of the Dorsal Vertebrae. 

 In Birds each dorsal vertebra is furnished with three, 

 and sometimes with four, prominent processes. Above the 

 neural canal is the neural spine, and on each side of it is a 

 transverse process (diapophysis) which articulates with the 

 shoulder (tuberculium) of the rib on each side. Below the 

 neural canal is the centrum, each end of which articulates 



2g 2 



