• mDIM TERTIARY & POST-TERTIARY VERTEBRATA. 
SIWALIK BIRDS. 
By R. LYDEKKEE, B.A., F.6.S., F.Z.S. 
(WITH PLATES XIY. AND XV.) 
INTRODUCTORY. 
Rarity of remains . — In the Siwaliks, as in most ossiferous formations, the remains 
of birds are extremely few as compared with those of mammals ; and such bones as 
do occur are generally in a fragmentary condition. This rarity is doubtless due to 
the small size of many birds, and also to the frail structure of the bones themselves,^ 
as well as to the circumstance that birds do not become ‘ mired.’ The absence of 
teeth in ordinary birds is another important element in the case ; since our knowledge 
of many tertiary mammals depends mainly, or entirely, on the evidence afforded by 
those organs. 
Classification . — Considerable difficulty is found in regard to the classification of 
fossil birds, since the characters on which the recent orders are usually founded are 
to a great extent external, or are of such a nature as to be inapplicable to fossils ; and 
the different orders do not consequently present the well-marked osteological 
differences of the Mammalia. Here again the want of the strong diagnostic 
characters afforded by the teeth of the latter is very strongly felt. In consequence 
of these wants it is frequently extremely difficult, or impossible, to say whether 
fossil birds belong to new orders or to those still existing ; and it may perhaps be 
doubted whether it will be found ultimately advisable to retain for palaeontological 
purposes many of the orders into which existing birds are divided. For the present, 
as a provisional arrangement, the writer has in the main adopted the classification 
given by Mr. Sclater in the “List of the Animals in the Gardens of the Zoological 
Society : the terms Carinatae and Ratitae, which are extremely convenient for 
1 This does not apply to the struthious birds. 2 8th ed., London, 1883. 
