lvon] the hares and their allies 3^3 



slightly forward like the spines of the lumbar vertebrae. The neural 

 spines of the remaining sacral vertebra become progressively smaller 

 from before backward and they are directed caudad. In some cases 

 the spine of the fourth sacral is very small. In nearly all cases 

 the spines of the sacral vertebrse are distinct and not fused with one 

 another. 



The outline of the sacrum as a whole is triangular, the base being 

 in front, the apex behind. The greatest width, which is at the 

 anterior part of the massive fused transverse processes of the first 

 and second sacral vertebrae, is nearly equal to the greatest antero- 

 posterior diameter. 



In all the rabbits the sacrum presents very few variations. The 

 differences found in the different sacra seem to be due entirely to 

 age and individual variations. The only sacrum showing any 

 marked deviation from the type is in Lcpus sp. No. 49,621, from 

 Jumna river, India. The posterior part of this sacrum is very 

 narrow from side to side. The expanded wing-like portions 

 to which the ilia are attached are very narrow from before back- 

 ward. In general, the shape of this peculiar sacrum, as seen from 

 below% is like a T. The adjacent tips of the first and second, and 

 of the third and fourth of its sacral neural processes are in contact. 



The usual number of vertebrae in the sacrum is four ; in some old 

 individuals it is raised to five, and in some younger ones it is only 

 three. In what follows with reference to the caudal vertebrae, four 

 vertebrae will be considered as entering into the formation of the 

 sacrum. 



Caudal vertchrcc. — The caudal series always includes vertebrae of 

 three different though not sharply defined forms. 



1. The first one or two vertebrae following the sacrum are long, 

 and in general appearance resemble the last sacral vertebra. 



2. The next three to seven cadual vertebrae are shortened, have 

 wing-like anteriorly directed transverse processes and the neural 

 arches become progressively less developed caudad. 



3. The remaining vertebrae of the caudal series, four to nine in 

 number, are merely small elongated centra without processes or 

 neural arches. 



The accompanying table (page 364) shows the number of the dif- 

 ferent forms throughout the caudal series in the available skeletons. 

 From this table the following groups may be picked out : 



1. Ncsolagus, with a total of eight caudal vertebrae. 



2. Ronicrolagus, with a total of nine caudals, of which one is of 

 the first form, five of the second, and three of the third. 



