POPULAR SCIENCE REVIEW. 
Dinosaurian skeleton is more characteristic than the sacrum 
(Fig. 6), which usually consists of a number of vertebrae blended 
together, so as to form a strong mass, to which short ribs 
were attached. These vertebrae are stated to be always more 
than two ; perhaps there may be no limitation in number. 
Certainly in Zanclodon there are only two sacral vertebrae 
united together. Thecodontosaurus, according to Professor 
Huxley, had three sacral vertebrae ; Hylaeosaurus not fewer 
than four ; Megalosaurus and Iguanodon five. In Anoplosaurus 
there are six, and in Agathaumus there are said to be eight 
or ten sacral vertebrae. Probably two i-s the primitive number, 
as among Crocodiles. And just as Professor Huxley has ana- 
lysed the chick sacrum into four true sacral elements, with 
dorso-lumbar elements in front and caudal elements behind, 
which have become blended together owing to the extension 
along them of the bones of the pelvis which are known as the 
iliac bones, so the extension of the ilium backward and forward 
in Dinosaurs, beyond the limits of the two original vertebrae, 
has caused the sacrum to increase in length, and thus to 
approximate in the number of bones it includes to the sacrum 
of a bird. But if it is bird-like at one end of the series it 
is crocodilian at the other; and here also we have evidence 
of a remarkable change going on in the skeleton of successive 
genera in past time, which I am inclined to regard as entirely 
functional and correlated with the circumstance that the earlier 
triassic Dinosaurs progressed in the manner of Crocodiles, 
while some of the later forms put so much of the energy of 
movement into the hind -limbs that they acquired a more or 
less erect mode of progression. The sacrum of Anoplosaurus 
is especially remarkable for the fact that its neural canal is 
enlarged so as to be wider than the entire centrum of a dorsal 
vertebra. 
The tail varies in length. Zanclodon Icevis has thirty-seven 
caudal vertebrae ; other Dinosaurs have probably many more. 
In the long- tailed genera, like Laelaps, Ceteosaurus, and 
Macrurosaurus, the later vertebrae become elongated into a 
dice-box shape. Perhaps the tail is shortest in Anoplosaurus 
from the Cambridge Greensand, in which only eight vertebrae 
have been found, and probably but few more existed. Many 
Dinosaurs, though not all, have in the first half of the tail 
Y-shaped bones articulated at the junction of each pair of 
vertebrae on the underside. Sometimes these chevron bones 
(see Fig. 7) are long, at others short; sometimes the V-shape 
becomes changed into a triangle by the articular facets uniting, 
as among certain lizards ; occasionally they are prolonged to 
the end of the tail. The tail shows less modification, perhaps, 
than any other part of the skeleton. As among Crocodiles the 
