MUSCLES OF THE HEAD 111 



The depressor mandibiihc consists of two parts which 

 have been termed digastric and hiventer. It connects the 

 under surface of the occiput with the lower jaw. 



Osteology ! 



Vertebral Column. Highly characteristic of birds is the 

 saddle-shaped (' heterocoelous ') form of the vertebral centra. 

 The existence of this mode of articulation, though confined 

 to birds, is not found everywhere and in all of the existing 

 members of the order, and is not found at all in some of the 

 extinct forms, Archceopteryx and Ichthyornis. As to exist- 

 ing birds, the opisthocoslous form is frequently met with, but 

 so irregularly as not to be of much use from a classificatory 

 point of view. The existence of such vertebrae, pointing 

 towards reptiles, may, however, be accepted as some indica- 

 tion of an archaic position in the order. The matter has 

 been lately summed up by the late Professor PARKER, 2 from 

 his own investigations, and from those of others. 



The dorsal vertebrae have been found to be opisthocoolous 

 in penguins and auks, in Limicolae (including Laridse), but 

 not in petrels ; among the parrots PARKER met with this 

 condition in several forms, where it was found to be com- 

 bined with terminal epiphyses, a character which is just 

 commencing in the lowest mammalia (OniitJwrJtynchus) and 



1 The following are a few works which deal with the general osteology of a 

 number of forms. More special treatises will be referred to in their proper place : 

 E. BLANCHARD, ' Recherches sur les Caracteres Osteologiques des Oiseanx,' 

 &c., Ann. Sci. Nat. xi. 1860, p. 11 ; J. F. BRANDT, ' Beitrage zur Kenntniss der 

 Naturgeschichte d. Vogel,' &c., Mem. Acacl. Sci. St. Petersb. (6), iii. 1840 ; 

 EYTON, Osteologia Aviuin, London, 1858-81 ; A. B. MEYER, Abbilchtngen 

 von Vogelskeleten, Dresden, 1879-96 ; MILNE EDWARDS and GRANDIDIER, Hist. 

 Nat., d~c., de Madagascar, ' Oiseanx,' Paris, 1879-85 ; C. L. NITZSCH, Osteogra- 

 jiltixclie Beitriicje, etc., Leipsic, 1811 ; P. GERVAIS, 'Description Osteologique de 

 1'Hoazin, du Kamiehi,' &c., Zool. in Voy. de Castdnau, Paris, 1855. Besides 

 FURBBINGEH, UntersucJiuiigcn zur Morphologic r. Syst. d. Vogel, and GADOW, 

 4 Aves,' in Bronn's Klasscnv. Ordniingen dcs Thicrreichs, for brief and largely 

 osteological definitions of birds see SEEBOHM, Classification of Birds, London, 

 1889, and SHARPE, Osteological Catalogue of College of Surgeons 

 London, 1891. 



2 ' On the Vertebral Chain of Birds,' Proc. Boy. S<>c. xliii. 1888, p. 465. 



