164 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. III. 



Measurements. Total length, 597; tail, 295; hind foot, 85; 

 ear, 21. Skull: occipito-nasal length, 67; Hensel, 60; zygomatic 

 width, 43; width of brain case posteriorly, 18; mastoid breadth, 

 26; interorbital constriction, 6.5; across postorbital processes, 

 23; interorbital width anteriorly, 11.5; length of nasals, 27; width 

 of nasals posteriorly, 2.5; width of nasals anteriorly, 8; greatest 

 width of rostrum, n; palatal arch to incisive foramina, 15; post- 

 palatal length, 22; length of upper tooth row, alveolar border, 

 15; total length of incisors from alveolus, 15; length of mandi- 

 ble, angle to upper edge of alveolus of incisor, 45; height at 

 condyle, 21, at coronoid process, 23; length of lower tooth row, 

 alveolus border, 15. 



This is apparently the largest species of muskrat known at 

 this time. It is remarkable for the peculiar formation of the 

 occipital region of the skull, the superior outline being greatfy 

 curved in the center, making a deep indentation at the inter- 

 parietal and of the same width as that bone; the brain case is 

 much contracted posteriorly, and the parietal is of a quite differ- 

 ent shape from that seen in F. zibethicus. The nasals, while 

 about the same length as those of the species just named, are 

 broader at their greatest width and are rounded at both ends; the 

 molars are smaller, and the mandible is deeper through than in 

 F. zibethicus. In color there is not much difference between the 

 present species and the common muskrat, but F. occipitalis is 

 somewhat darker above and redder beneath. *omF.spatulatus 

 this form differs in many ways. It is much larger, has very 

 differently shaped nasals; the parietal much wider and longer, 

 the interparietal higher and rounded anteriorly, and the occipital 

 outline deeply bowed in center, which feature appears entirely 

 absent in type of F. spatulatus.* 



GENUS RHITHRODONTOMYS. 

 Rhithrodontomys peninsulae. Sp. nov. 



Type locality. San Quentin, Lower California, Mexico. 

 Geogr. distr. Only known from type locality. 

 Gen. char. Tail less than half the total length; size large; 

 pelage more red than that of R. longicaudus, with longer tail and 

 hind foot, and much larger ears. Skull larger, nasals longer, 

 brain case broader; interorbital space of nearly equal width 

 throughout, sides nearly parallel. 



* See PI. VI, Fig. 4, N. Am. Faun., No. 19, 1900. 



