552 MONOGRAPHS OF NORTH AMERICAN RODKNTIA. 



of the Sciurine series. Briefly, then, I at present accept a superfamily group 

 Sciuromorpha in the sense lately attached to it by Mr. Alston, as including 

 the genus Sciurus and its unquestioned allies, as well as Haplodon, Ca$tor,ari(l 

 Anomalurus, with which latter I am acquainted only by descriptions. Since 

 Waterhouse, many years ago, foreshadowed a more refined classification of 

 the Rodents by his four families of Sciurida, Murida, Hystricid?, and 

 Leporidee, there has been a close general agreement among leading writers 

 that these groups, whatever their absolute rank, represent as many natural major 

 divisions of existing Rodents. The Leporidee, by nearly common consent, are 

 now considered as one of two primary divisions of recent Glires, as such 

 comparable in value to all the families of "simplicidentate" Rodents com- 

 bined. The SciuridtB, Muridce, and Hystricidce of Waterhouse, with whatever 

 modification in details, yet stand as indices of groups of Rodents, of whatever 

 value we may assign, the members of each of which are much more nearly 

 interrelated than any one of them is to any member of either of the other 

 groups. In the paper already several times cited, Mr. Alston seems to me to 

 have defined the three groups, which he calls simply "sections", in a very 

 satisfactory manner; and he certainly lias given us an easy means of distin- 

 guishing them. "Even if it were not possible to separate the first three of 

 Waterhouse's great families by perfectly constant characters," says Mr. Alston, 

 "they ought, as it appears to me, to be recognized as indicating three distinct 

 lines of development. But by the help of the characters of the leg-bones, 

 pointed out by Professor Lilljeborg, the difficulty is overcome. In the few 

 eases in which the cranial differences fail us in separating the Sciurine rodents 

 from the Murine, and the latter from the Hystricine, the complete anchylosis 

 of ihe lower part of the tibia, and fibula in the second group comes to our 



aid The first and third groups, which agree with one another in this 



point [distinction of fibula], are at once separated from each other by the form 



of the mandible, as well as by the whole type of cranial structure 



The first section, Sciuromorpha, lias for constant characters the combination 

 of a peculiar form of mandible with the persistence of the fibula as a distinct 

 bone throughout life. The former character at once separates it from the 

 ffystricomorpha, the latter from the Myomorpha? This is the sense, then, 

 in which I am to be understood to accept the Sciuromorpha, in my present 

 reference of the Haplodontida to that series as one of its component families, 



