No. 406.] DINOSA UR-A VIAN STEM. 789 
Thus, in two of the largest Cetiosauria or Sauropoda we 
have an extremely short back, resembling the short back of 
birds, also an extremely long flexible neck, a very rigid attach- 
ment between the sacrum and ilium, correlated with the power 
of temporarily raising the entire presacral portion of the body. 
The significance of these avian resemblances in the neck 
and trunk of these gigantic dinosaurs is rather homoplastic 
than genetic, for the peculiar paired cervical and cervico-dorsal 
spines, the posterior abdominal ribs, the lengthened pre- 
acetabular iliac bar, correlated with certain feeding motions, 
are bird-like structures mingled with other non-bird-like struc- 
tures too numerous to mention. So also with the resemblances 
among the bipedal dinosaurs, in which the presacral portion 
of the body is permanently raised, bird-like and non-bird-like 
structures appear in close propinquity. 
The main avian character pervading all Dinosauria is the one 
originally observed by Gegenbaur, namely, the close junction 
of the astragalus with the tibia or tendency to form a tibio- 
tarsus. However, where there is so much smoke there may 
. be some fire, and we may now proceed to look into the proba- 
bility of the existence of a primitive bipedal dinosaur-avian 
ancestor. 
V. Tur CLAWED QUADRUPEDAL ANCESTRY OF BIRDS. 
Pycraft (96, p. 261) has recently discussed’ with care the 
osteology of Archaeopteryx. In opposition to the view of 
Hurst, that the manus retains five digits, two of which were 
used in climbing trees, Pycraft supports the older view, that 
digits I, II, III are the only ones represented, and that digit 
III, as in the Archosauria! generally, and in the Dinosauria 
in particular, had four phalanges, the terminal of which was 
armed with a claw. In addition to these reptilian characters 
are the thecodont, or socketed teeth, the flat, or amphiplatyan, 
! * Archosauria " is a term employed by Cope for reptiles with two cranial 
arches at the back of the skull, namely, Rhynchocephalia, Crocodilia, and Dino- 
sauria. In the writer's opinion this group should be extended to include the 
Lacertilia, in which one arch has been lost. — - 
