THE DINOSAURIA. 
The pelvis, on the other hand, is unlike that of any reptile. 
As is well known, the Ilium, or hone which extends along 
the sacral vertebrae, is in Reptiles either vertical or directed 
backward, though it attains its most expanded form in the 
Crocodiles (Fig. 11). In Birds it extends about equally before 
and behind the acetabulum, or cup for the articulation of 
the femur. In the Mammals it is directed almost entirely in 
front of the acetabulum. Too much importance ought not to 
be attached to the direction of this bone as a mark of affinity, 
because its direction is almost always entirely forwards in frogs, 
probably without any other explanation than the influence of 
the manner of progression of the Anura in determining the 
direction of its growth. Similarly we are as little justified in 
regarding the Avian extension of the Dinosaurian ilium as a 
mark of Avian affinity in these animals, as we should be in 
regarding the direction of the Anurous ilium as an evidence of 
Mammalian affinity. When the shape of the ilium in Triassic 
Dinosaurs is examined and compared with that of the Crocodile, 
the resemblance is closer than might have been anticipated. 
The ilium has its chief extension backward ; there is a process 
directed downward to the ischium, and forward to the pubis. 
This is well seen in Zanclodon, as is proved by specimens in 
the Tubingen collection ; and I infer by analogy that the left 
ilium of Thecodontosaurus figured by Professor Huxley also 
had its chief extension behind the acetabulum (Fig. 12). But 
there is no doubt that other genera, like Iguanodon (Fig. 13), 
had the anterior process of the ilium, which is not developed 
in Thecodontosaurus, extended far forward ; while other genera, 
like the Morosaurus of Marsh (Fig. 10), show the bone to have 
great depth, and, if rightly interpreted, to have its chief 
direction inclining anteriorly. There would thus appear to 
have been a progressive modification of this bone, by which 
a reptilian form which was associated with a Reptilian mode 
of progression became changed for an Avian shape in animals 
in which the progression was birdlike. And, although I con- 
fess to doubts concerning the right reading of some of Professor 
Marsh’s figures, it would appear that these changes of form 
tended slightly towards the Mammalian pattern. The ischium 
is essentially a Crocodilian bone ; and in the Atlantosaurus or 
Camarasaurus figured by Professor Marsh has a shape which is 
Crocodilian, meets the pubis much in the same manner as 
in the Crocodile, and the bones appear to have a Crocodilian 
direction. But, on the other hand, in some other genera, 
such as Iguanodon, ischium and pubis are slender bones, which 
resemble, both in form and direction, the same elements in the 
Apteryx or the Emeu ; and this resemblance is the more 
interesting since Mr. Hulke has shown that the pubis developes 
