552 



MONOGRAPnS OP NORTH AMERICAN RODENTIA. 



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(>r the Sciurino «cri(;s. lliiefl}', tlicn, I at present ncccpt n superiainily grouii 

 Sciuromorpfia in I lie sense lately attached to it by Mr. Alston,- as including 

 the genus Sciuruis and its unquestioned allies, as well as Haplodon, Castor, and 

 Anoma/uru*, with which latter I am acquainted only by descriptions. Since 

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

 the Rodents by his four families of Sciurida, Murida, Hystridda, and 

 Lejioridce, 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 Kodcnts. The Lcporidtt, 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 "sim])iicidentate'' Rodents com- 

 bined. The Sciurida, Murida, and Hystricida 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 has given us an easy means of distin- 

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

 Watcrhouse's great families by |>erfectly constant characters," says Mr. Alston, 

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

 )ine8 of development. But by the help of the characters of the leg-bones, 

 pr'iited out by Professor Lilljeborp, the difficulty is overcome. In the few 

 cases in which the cranial diiferences fail us in separating the Sciurine rodents 

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

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



ai<l 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 secion, Sciuromorphn, has 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 

 Hystiicomorpha, 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 Haplodonlida to that series as one of its component fiimilies. 



