176 Mr. O- Thomas on Indigenous 



of the interparietal bone, whicli is about an inch deep, broken 

 on each side, has a vertical median ridge, and at its base 

 shows a fractured fragment of the supraoccipital, which is 

 excavated on its anterior border by a vertical concavity like 

 that seen in figured Dicynodonts in the British Museum, 

 which display the cerebral aspect of the occipital plate. The 

 bone is obviously narrow ; at its sides are the tiiin walls of 

 the brain-case ; those walls meet inferiorly in the median line, 

 so as to rest upon the interorbital septum which has been 

 described ; and it extends backward to the supraoccipital and 

 interparietal bones, but not much in advance of the anterior 

 angle of the bevelled concave temporal region, where the 

 postfrontal rests on the parietal. The parietal bones form the 

 upper covering of this brain -case. 



All the allied skulls which I have seen from the Upper 

 Karroo rocks are remarkable for side to side compression, 

 while the Ptychognathus type widens superiorly to the flat 

 interorbital table on the top of the head. 



XXVII r. — On Indigenous Murida3 in the West Indies ; with 

 the Description of a new Mexican Oryzomys. By Old- 

 FiELD Thomas. 



In Mr. F. M. Chapman's interesting paper on the origin of 

 West-Indian bird-life * it is assumed that there are no indi- 

 genous terrestrial mammals in the Greater Antilles other than 

 Solenodon, Plugivdontia, and Capromys, or in the Lesser 

 Antilles than Dafiijprocta cristata (to which should be added 

 Megalomys piloridts) . 



i^or more than half a century, however, there lias been in 

 the British Museum a jat from Jamaica belonging to the 

 genus Oryzomys^ and closely allied to the Central American 

 O. Couesi, while another indigenous species has now turned 

 up in a member of tiie same genus from St. Vincent, collected 

 by Mr. H. H. Smith about six years ago, but hitherto over- 

 looked. 



In view of the fact that, as is evidenced by their rarity, 

 these indigenous Murines are rapidly disappearing before the 

 competition of the introduced European rats and mice, these 

 specimens aie of much interest as furnishing valuable evidence 

 about the character of the original West-Indian fauna. 



* " Notes on Birds and Mammals observed near Trinidad, Cuba, with 

 Remarks on the Origin of West-Indian Bird-life," Bull. Am. Mus. X. H. 

 iv. p. 279 (1802). 



