1885.] MISS B. LINDSAY ON THE AVIAN STERNUM. 711 



Tabular Classification of parts of the Sternum 



A. Part common to Sauropsida and Mammalia : — 



Costal sternum, arising in two bands ; connected with sternal 

 ribs in the adult, but olteu losing its connection with ribs 

 which took part in its early formation. 



B. Part common to Ratitce and Carinatce, but seen to be wanting 

 in early embryos of the former, never of the latter; it is thus 

 seen in process of formation in the former, as indicated by its partly 

 cartilaginous condition in the adult : — Metasternum. 



C. Part apparently common to both Eatitae and Carinatse, but really 

 of different origin : — 



Anterior lateral process ; seen added to the costal sternum in 

 the Ostrich, formed by atrophy of anterior ribs in the Chick 

 and Gannet. 



D. Part absent in Ratitse, but common to all Carinatae : — 



Keel ; the median ventral outgrowth of B. 



E. Part common to some Ratitse (Ostrich, and indicated in Rhea) and 

 tiie majority of the Carinatse ; — 



Posterior lateral process. 



F. Parts variant in families and genera of Carinatae : — 



Accessory processes of Metasternum. 

 Eoslrum or anterior median sternal apophysis. 

 Xiphoid ends of posterior processes. 



Variations of the Posterior Lateral Process. 



The posterior lateral process of the sternum, if the value above 

 assigned to it be correct, ought to be found a constant feature of the 

 sternum of birds. Comparison of types shows that this is the case : 

 the birds that have a sternum without processes are few, the most 

 familiar instances being the Crane and the Goose. It must not, 

 however, be supposed that whenever we find a posterior lateral 

 process, this represents the primitive process as seen in the Ratitse ; 

 for example, we find in the Spheniscidse, &c. {cf. figures given by 

 Watson, 'Challenger' Report on the Spheniscidse, and by Forbes, 

 Report on the Tubinares), variations in the shape of the posterior 

 processes which suggest that they have been formed by completion 

 of the growth of the sternum. In such cases, however, the primitive 

 process must form a part of the new one. 



A secondary process exists in a large number of birds ; and the 

 study of development has already shown that such an accessory process 

 may be added either on the external side of the primitive postero- 

 lateral process, as in the Chick, or on the inner side, as in the Gull. 

 In the Pheasants, as well as in the Gallinaceae, it is doubtless formed 

 as in the Chick : in many other birds, for example in the Bustards, 

 Curlews, Plovers, &c., its exact origin is at present unknown. 



It is worthy of remark that the accessory process, when present at 

 all, is not conspicuous in birds of archaic types, while the large 



