14f) MONOUUA1MIS OF NORTH AMERICAN RODENTIA. 



The foregoing table is made up of measurements published by Baird in 

 1857, with those of twelve additional specimens. The dry measurements are, 

 of course, only approximately correct, and, as far as the total length of body 

 is concerned, are a little over the truth, from over-stuffing, as is certainly the 

 case with No. 1092. Probably no one of them was in life over 4.00 at the 

 outside, and the real average cannot be over 3.50, instead of 3. GO, as the fig; 

 ures stand No. 2872, which we have not seen, is most likely ungrown. 

 These circumstances tend to bring the maxima and minima a little nearer 

 together, say 4.00 and 2.75 for total length, &c. 



On comparing this table with that of A. rutilus, it will be seen that the 

 average size is greater; that the tail- vertebrae average about a third of an 

 inch longer, and the tail with its hairs little if any longer, showing the great 

 difference in the length of the terminal pencil; the foot, is 0.72 instead ot 

 about 0.70 on an average. The tallies also show that while gapperi touches 

 figures that rutilus rarely reaches, and that the average of the latter is near 

 the minimum of the former, especially as regards tail, feet, and ears, that 

 nevertheless the intergradation is complete. 



Description of the skull and teeth of A. gapperi. — Aside from the generic 

 features given under head of Evotomys, the skull at gapperi does not differ very 

 noticeably from that of Arvicola in general. It averages in length 0.95, by 0. r )2 

 in zygomatic breadth, or about as 100 to 55. The interorbital constriction is 

 about as broad as the rostral portion of the skull. The molar series is one-fifth 

 or barely more of the total length. The upper incisors protrude a little less, and 

 the under a little more, than the length of the molar series. In the lower jaw, 

 the distance from the tip of the incisors to the end of the hamular process 

 equals or is even less than the distance from the same point to the back of the 

 condyle. This is as in Pitymys, and not as in the riparius section of Arvicola, 

 where the former measurement exceeds the latter. The height of the skull, 

 measured from the last molar inclusive perpendicularly upward, is just about 

 one-third of the length. The interparietal bone is acute-angled laterally; 

 there is a little foveole on the frontal; the nasal branch of the premaxillary 

 is not longer than the nasal bone, and neither extends back of the anterior 

 loot of the zygoma. The tympanic bullae are very much inflated and papery; 

 the foramen magnum is large and subcircular. The incisive palatal foramina 

 are long and narrow; the anteorbital are as usual in the subfamily. In adult 

 skulls, the muscular impressions are distinct, leaving a shield-shaped plateau 

 on too of the skull. 



