370 



premolar has not yet begun to be calcified. The right ramus has been transversely 



bisected. 



Presented by Prof. Owen, F.R.S. 



Family Dasyproctidee (Pacas and Agoutis). 

 Genus Ccdoyenys. 



Dental formula : % ~, p j^, m ^=20. 



2041. The skeleton of the Paca (Cadoycnys Paca). 



The vertebral formula is: 7 cervical, 13 dorsal, 6 lumbar, 4 sacral, and caudal. The 

 transverse processes of the last cervical are imperforate. Seven pairs of ribs articulate 

 directly with the sternum, which consists of six bones. The anapophyses and metapophyses 

 begin to be developed on the seventh dorsal, in which their potential base extends above the 

 diapophysis as a continuous broad ridge ; in the eighth dorsal the ridge is bilobed ; in the 

 ninth the lobes diverge; in the tenth and eleventh they become distinct processes: the 

 diapophyses maintain their individuality in all these vertebrae. The anapophysis disappears 

 on the last lumbar, hut the metapophysis is retained, as in all the other lumbars. The pleur- 

 apophyses or rudimental ribs are retained at the ends of the long antroverted diapophyses of 

 the last two lumbar vertebrae. The supraspinal fossa is deeper than the infraspinal one. 

 The acromion is of great length, and bifurcated, but the lower branch is not developed as in 

 the Hares. The humerus is perforated between the condyles, but not above the inner con- 

 dyle. Both fore and hind feet are pentadactyle ; the pollex is short and with two phalanges ; 

 but the hallux, in this skeleton, has three phalanges like the other toes, and is nearly equal in 

 length to the second toe. There are two supplementary ossicles on the inner side of the 

 tarsus, one articulated to the astragalus and scaphoides, the other to the entocuneiform and 

 first metatarsal. There is a third larger supplementary ossicle beneath the tarsus ; the ecto- 

 cuneiform is articulated to the cuboid, not to the scaphoid. The size of the cranium and the 

 degree of rugosity upon its upper surface, and especially upon the outer surface of the enor- 

 mously expanded zygomata, indicate this specimen to have belonged to a male. 



Hunterian. 



204.2. The skull of a Paca (Ccelogenys Paca). 



The teeth are removed from the left side of both upper and lower jaws, and are separately 

 displayed. From its smaller and more slender dimensions and minor development of the 

 zygomatic arches and of the external rugosities, this skull has probably belonged to a female. 

 It is adult with the mature dentition, and the sutures between the elements of the occipital 

 are obliterated, as is likewise the sagittal suture. There is no trace of interparietal bone. 

 The petrotympauic is free from the squamosal, and rather loosely suspended beneath the 

 overarching posterior lamella of the squamosal, which bends down external to the mastoid 

 and paroccipital. The malar is a slightly curved plate, twice as deep as it is long, and forms 



