3^^ SMITHSONIAN MISCELLANi>-OUS COLLECTIONS [vOL. 45 



The xiphisternum is usually the longest single segment. It is 

 attached anteriorly to the last mesosternal segment. Its posterior 

 end is free and is terminated by a thin rounded piece of cartilage. 



Eight different types of sternum are found among the Leporidse ; 

 some of which are characteristic of certain genera, while the others 

 are not so sharply defined and might better be considered tendencies 

 rather than actual developments. A greater number of. skeletons 

 may show that some of the types amount to nothing, as there seems 

 to be a certain amount of variation in individuals of the same species. 

 The different types of sterna are as follows : 



1. Presternum long and narrow, much compressed laterally into a 

 keel which is most prominent anteriorly ; the first rib is attached at 

 the junction of the first and second fourths or first and second 

 thirds. Mesosternum of four distinct segments, of which the first 

 is narrow and compressed, the others not compressed or even flat- 

 tened dorso-ventrally. All the mesosternal segments in general are 

 subequal and each successive one grows wider from before back- 

 ward. The xiphisternum about equals the presternum in length. 

 The anterior end is considerably enlarged where it articulates with 

 the last mesosternal segment. Its posterior and free extremity is 

 rather pointed or only slightly larger than the narrowest portion of 

 the xiphisternum. One skeleton (Lcpiis fcxianiis, No. 94,198), be- 

 longing in this section, has an anomalous mesosternum composed 

 of five distinct segments, the last of which is only slightly smaller 

 than the other segments. At the articulation of the mesosternum 

 with the xiphisternum, but one pair of ribs is attached instead of the 

 usual two pairs in sterna where the fifth mesosternal segment is 

 normally suppressed. 



The hares possessing sterna like the above are all large and belong 

 to the genus Lcpus. (PI. xcv, 3, 4.) 



2. Sternum in general very similar to the above, presternum re- 

 latively longer, its keel less prominent anteriorly. The first pair of 

 ribs is attached just anterior to its middle. The mesosternal seg- 

 ments have a tendency to be less flattened. Xiphisternum shorter 

 than the presternum, its posterior end more enlarged than it is in the 

 case of the preceding section. Such sterna are more or less charac- 

 teristic of the genus Sylvilagus. (PI. xcv, 2.) 



3. Sterna similar to those of the genus Lcpus; presternum more 

 conspicuously keeled, the first rib attached just anterior to its middle. 

 Mesosternal segments more compressed from side to side than they 

 are in Lcpus. The last is much shorter than any of the other meso- 

 sternal segments. Xiphisternum large and stout, longer than the 



