198 Mr. G. R. Waterhouse on the Rodentia. 



The Psammoryctina of Prof. Wagner I have long regarded 

 as a natural group, and the genera of which it is composed 

 were originally thrown together in the table before alluded to. 

 The principal characters which induced me to remove those 

 genera from other groups in which they had previously been 

 placed, are pointed out in the introductory observations to my 

 paper on the Rodents published in the ' Magazine of Natural 

 History 5 for February 1839. 



In the table published in the ( Proceedings/ however, there 

 are two genera which are placed in juxtaposition with the 

 others composing Prof. Wagner's family Psammoryctina, and 

 which that author does not include in the family ; I allude to 

 the genera Dasyprocta and Ccelogenys ; on these I shall have 

 to make some observations hereafter, as well as upon the little 

 section of the family last mentioned, to which I gave the 

 name Octodontidce in the f Zoology of theVoyage of the Beagle/ 



The tenth family of Prof. Wagner's classification, the Hy- 

 stricina, I have with most other naturalists regarded as a na- 

 tural group, and have restricted to the same limits in the 

 table. 



The remaining three families, according to the distribution 

 of the Rodents under consideration, are, Cunicularia, Casto- 

 rina and Subungulata ; under these three heads Prof. Wagner 

 has grouped together various genera, as it appears to me, in an 

 unnatural manner. 



Before however I proceed to the consideration of this por- 

 tion of the subject, which is the chief object of these observa- 

 tions, I may perhaps be permitted to take a short review of 

 my arrangement of the order. 



At the head of the Rodents are placed the Squirrels (Sciu- 

 ridce), which have the largest cranial cavity, the smallest deve- 

 lopment of the facial bones, and the most perfect palate ; the 

 sphenoids are broad and well- developed, and the posterior 

 openings for the transmission of nerves from the brain are 

 proportionately small. 



In the highest mammals (Quadrumana) it is that we find 

 the most perfect bony orbit for the eye, and it is in the Sciu- 

 'ridce among Rodents that we find the orbit also most perfect. 

 In their frugivorous diet and arboreal habits they may also be 

 compared to the Quadrumana. In their dentition, moreover, 

 the present animals evince a superiority over their congeners. 



The extremities of the Squirrels are fitted for climbing and 

 running, and their clavicles are well-developed. They differ 

 from nearly all other Rodents in wanting a character which is 

 observable only in this order ; — I allude to an ant-orbital open- 

 ing for the transmission of a portion of the masseter muscle. 



