Zl'^ SMITHSONIAN MISCELLANEOUS COLLECTIONS [vOL. 45 



RIBS 



Leporid.'E 

 (Plate XCIV, 1-8) 



Corresponding to the number of thoracic vertebrae there are 

 twelve pairs of ribs of which seven are normally attached to the 

 sternum by means of well developed, ossified, costal cartilages. The 

 last three or four pairs of ribs have no ventral attachments. The 

 ribs intervening betw^een the sternal and floating ribs are attached by 

 their costal cartilages to the costal cartilages of the sternal ribs. 



The typical rib has a head attached to the centra of two adjoining 

 vertebrae, a well-marked tubercle a portion of which articulates with 

 the transverse process of the more anterior vertebra and another 

 portion of which projects superiorly, forming a spine. The most 

 posterior ribs lack the tubercle. The first rib has the articular 

 portion of the tubercle well developed but the spine-like part is 

 lacking. Its place, however, is taken by a dorsally projecting spine 

 from the transverse process of the first thoracic vertebra. The rest 

 of the rib is made up of a long, thin, flattened shaft. 



In the genus Lcpns the non-articular portion of the tubercles are 

 very well developed, the eighth pair of ribs is the last pair bearing 

 them. The second, third, fourth, fifth, and sixth ribs are very flat 

 and broad in the ventral half of the shaft ; the greatest width of 

 the rib just behind the tubercle is very much less than the width 

 of the shaft in the lower portion. The sternal costal cartilages in 

 Lcpns are very short and wide, as compared with the sternal costal 

 cartilages of the other genera. 



In Oryctolagus conditions similar to those found in Lcpns exist 

 regarding the ribs, but the shafts of the anterior ribs are relatively 

 less widened. 



The genus Syh'ilagus closely resembles Lcpns with respect to the 

 form of the ribs, but like Oryctolagus the shafts of the ribs are less 

 expanded and in some of the species, notably S. uiincnsis, the non- 

 articular portions of the tubercles are not so well developed and are 

 last seen on the seventh pair of ribs. 



Brachylagns closely resembles Sylvilagns, especially 6". uiincnsis; 

 the non-articular portions of the tubercles are not very well developed 

 and are last seen on the eighth pair of ribs. 



In LivDiohgns the tubercles are well developed but are not con- 

 spicuous, owing to the fact that the angle between the tubercle and 

 the posterior edge of the rib is filled up with bone, making that 

 part distinctly the widest portion. In the single skeleton at hand 



