ARSrNOlTIIEEIUM. 
31 
portion the centrum may bo nearly twice as wide as it is high ; at the same time 
it becomes somewhat more elongated. The neural border of the centrum is 
straight and the neural arch is low and wide. The zygapophysial surfaces are 
cylindrical and the neural spine is low and projects considerably behind the centrum, 
while at the same time the anterior border of the arch leaves the anterior half 
of the neural surface of the centrum exposed. The transverse processes are broad 
from before backwards, and in the anterior region they are narrow from above 
downwards, but as they are followed backwards they become larger and more massive. 
In the whole of the large collection of vertebrae in the Museum at Cairo there 
is not a single example of the sacrum, nor does any such occur in the collection 
in the British Museum. On the other hand, there are a considerable number of 
vertebrae of the form shown in text-fig. 14: these greatly resemble the lumbar 
vertebrae, except that the lateral processes are very large and massive, and bear 
at their extremities broad, roughened, nearly triangular surfaces (il.s.), apparently 
Sacral (?) vertebra of Arsinoitherium zitteli : A, from front ; B, from right side. 
a.Z; anterior zygapophysis ; n.s^j., neural spine ; il.s., iliac surface. 
a.z. 
Text-fig. 14. 
for union with the ilium. These vertebrae are believed to be the anterior sacrals. 
Another somewhat different type was probably the second sacral : in this the 
centrum is even more depressed than in the last, and its ventral surface is almost 
in the same plane as the ventral surface of the lateral processes, which are very 
long and terminate in a surface which looks somewhat backwards. The fact that, 
so far as at present known, the sacrals do not unite as usual to form a sacrum is 
especially remarkable in an animal of such size and weight. Of course, it is possible 
that a true sacrum may yet be found, but considering the usual solidity of that 
structure it is very curious that in so large a collection no specimen should occur, 
while a score or more examples of the atlas and axis have been obtained. 
The anterior caudals have a very depressed centrum, with broad, flat, rather long, 
transverse processes. The neural arch is low and situated on the posterior half of 
the centrum. Further back the caudal centra become more cylindrical, with short 
