340 O. C. Marsh—JTurassice Birds and their Allies. 
opteryx, and the two forms are in reality widely separated. 
While examining the Compsognathus skeleton, I detected in the 
abdominal cavity the remains of a small reptile which had not 
been previously observed. The size and position of this in- 
closed skeleton would imply that it was a foetus; but it may 
possibly have been the young of the same species, or an allied 
form, that had been swallowed. No similar instance is known 
among the Dinosaurs. 
A point of resemblance of some importance between birds 
and Dinosaurs is the clavicle. All birds have those bones, but 
they have been considered wanting in Dinosaurs. Two speci- 
mens of Jguanodon, in the British Museum, however, show that 
these elements of the pectoral arch were present in that genus, 
and in a diagram before you one of these bones is represented. 
Some other Dinosauria possess clavicles, but in several families 
of this subclass, as I regard it, they appear to be wanting. 
The nearest approach to birds now known would seem to be 
in the very small Dinosaurs from the American Jurassic. In 
feathers, as I have shown in my Memoir on the Odontornithes, 
published during the past year. 
It is An interesting fact that all the Jurassic birds known, 
both from Europe and America, are land birds, while all from 
the Cretaceous are aquatic forms. The four oldest known birds, 
moreover, differ more widely from each other than do any two 
recent birds. These facts show that we may hope for most 
important discoveries in the future, especially from the Triassic. 
which has as yet furnished no authentic trace of birds. For 
the primitive forms of this class we must evidently look to the 
Paleozoic. 
