796 THE AMERICAN NATURALIST. [Vor. XXXIV. 
gradation of Chlamydosaurus, induced by the great relative 
shortness of the first and fifth digits, is distinctly indicated in 
Fig. 1 of the plate previously referred to.” 
Thus, tridactylism is correlated with rapid bipedal progres- 
sion, the inner and outer digits suffering reduction. In fact, 
a glance at the digital formula of Archosauria shows why tri- 
dactylism is a likely resultant of rapid digitigrade progression. 
teeta ak II III IV V 
Number I I I I I 
of 2 2 2 2 2 
Phalanges 3 3 3 3 
in 4 4 
Archosauria 5 
There is considerable ground for regarding a certain degree 
of bipedalism as a character common to all dinosaurs. Among 
the carnivorous Megalosauria there can of course be no ques- 
tion, because this condition 
marks the oldest Triassic 
types. Osborn, in Diplodo- 
cus, has demonstrated the 
truth of Cope's conjecture 
that the quadrupedal cetio- 
saurs occasionally rose upon 
their hind limbs. Among 
ion XEM sm d iet uo ap ham dp Aa the Predentata, the Iguano- 
dontia are typically bipedal, 
and the iguanodont or quadriradiate structure of the pelvis in 
the quadrupedal Stegosauria and Ceratopsia has led Dollo to 
advance the somewhat daring hypothesis that these animals also 
were at one stage more or less bipedal and that their fixed 
quadrupedal habit is possibly secondary. 
VIII. HYPOTHETICAL ORIGIN or BIRDS FROM A PRIMITIVE 
BIPEDAL DINOSAUR. 
If bipedalism subsequently proves to be a common dinosaur 
character, it would naturally strengthen the dinosaur-avian 
stem hypothesis. The presence of a free guadrate in birds, a 
