NATURAL SCIENCES OF PHILADELPHIA. 273 



to the Saccomyinse of Baird, 1857, and of Gray, 18G8, which is 

 placed next to the Geomyidse, with which it constitutes one of 

 this author's numerous taxonomic refinements, namely, a "super- 

 family" Saccomyoidea. These groups, however, are merely named, 

 no characters being furnished for their recognition. Use of the 

 term " Saccomyoidea" seems to me to be one way of expressing 

 the undoubted fact that the Saccomyidse and Geomyidse are more 

 nearly related to each other than either of them is to the Muridse 

 or Castor'idse, between which they are interposed in Dr. Gill's 

 system ; through the practical utility of such " super" groups 

 would be questioned by some naturalists. 



Thus determining a family Saccomyidse (characters of which 

 will presently be given), we may, before proceeding further, note 

 what disposition has been made of its component genera. Dr. 

 LeConte, in 1853, and Prof. Baird, both treated only of North 

 American forms, recognizing simply two genera, Dipodomys and 

 Perognathus (-j- subg. Cricetodipus). Dr. Gill made no subdi- 

 vision of the family. Dr. Gray went into this matter by dividing 

 his Saccomyinse (== Saccomyidse) into two groups of no assigned 

 value, namely, Dipodomyina, with Dipodomys alone, and Hetero- 

 myina, to include all the other genera. This division is based 

 primarily upon the state of the molars, whether rootless (Dipo- 

 domys) or rooted. The Heteromyina of this author are further 

 divided into those with sulcate superior incisors (Perognathus, 

 " Abromys," and Cricetodipus) and those in which the upper 

 cutting teeth are smooth (Heteromys and Saccomys). 



I am insufficiently acquainted with Heteromys and Saccomys 

 to say how nearly they approach, or how far they recede from, the 

 Perognathi with which I am familiar ; nor have I at present the 

 material to inform myself. Of Saccomys I do not find that any 

 one has precise knowledge at present ; the very habitat of the 

 animal is uncertain. To the orginal Heteromys anomalus (Mus 

 anomalus, Thomps., Linn. Trans. XI. 151, pi. 10 fide Gray) Dr. 

 Gray added several others in 1868, all from Middle America. As 

 this author draws his characters, Avith the exception of those of 

 the front teeth, entirely from some trivial superficialities which 

 may be observed upon looking at stuffed skins, I am unable to 

 gain from his remarks any satisfactory opinion respecting the 

 degree of relationship subsisting between Perognathus and Crice- 

 todipus on the one hand, and Heteromys and Saccomys on the 



