84 MB. II. G. SEELEY ON THE 



On the Organization of the Ornithosauria. By Ha ret Gotieb, 

 Seelet, F.L.S., P.Z.S., F.Gr.S., Professor of Geography in 

 King's College, London, 



[Bead November 18, 1875.] 

 (Plate XI.) 



The Pterodaetyles have long been grouped with the Eeptilia by 

 the chief European anatomists. De Blainville placed them as a 

 class between reptiles and birds. Earlier writers (Hunter and 

 Blumenbach) , with slender materials, included these animals with 

 birds ; and others, like Sommerring, believed that they had some 

 affinities to mammals. These conclusions necessitate modifica- 

 tions of the zoological ideas of reptile and bird on the part of 

 those who would place Ornithosaurs in either group. But the 

 evidence as to their organization has not been very conclusively 

 set forth ; and their place in nature has always been more as- 

 sumed than proved. The difficulty consists chiefly in the absence 

 from anatomical science of definitions which would fix the zoolo- 

 gical value of the characters observed in such fossils as these. 

 No one has specified with sufficient detail the osteological struc- 

 tures which constitute an animal a reptile, or a bird. The task 

 is extremely difficult. My own endeavours that way have led to 

 the conclusion that it is within the limits of possibility for an ani- 

 mal to have its skeletal characters so modified by loss, substitution, 

 or development as to be no longer recognized as a member of its 

 class by the form or proportions of a single bone. The great 

 range of actual variations in the skeleton, seen among fishes, rep- 

 tiles, and mammals, sufficiently demonstrates that characters 

 must be found more constant than those of the bones before an 

 extinct animal's affinities can be indubitably determined. There- 

 fore, though ordinal groups are defined by the bones without diffi- 

 culty, the characters of the class can only be found in the soft 

 vital organs. 



In the animals whose organization I purpose to examine, two 

 of the vital organs can be investigated — one by the form of the 

 cerebral cavity of the skull, the other by nearly all the bones 

 showing conspicuous apertures which are formed and situate pre- 

 cisely as are the pneumatic foramina in the bones of birds. Such 

 foramina characterize no other kind of skeleton ; and since in birds 

 they serve to prolong the air-cells from the lungs into the bones, 

 it can only be inferred legitimately that the similar foramina in 

 fossil bones subserved an identical purpose. 



