

it 



146 



MONOOKAPUS OF NORTH AMERICAN RODENTIA. 



Tlie forogding fiiblc is inndfi up of measurements publislicd by Bnlrd in 

 1857, witli tiiose of twelve ndditionnl specimens. Tlie dry measurements are, 

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

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

 case with No. 10!)2. T'- >bly no one of them was in life over 4.00 at the 

 outside, and the real av ;n.ge cannot be over 3.50, instead of 3.60, as the fig- 

 ures stand No. 2872, whiclv we have not seen, is most likely ungrovvn. 

 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. ruli/us, 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 iiairs little if any longer, showing the great 

 •lifference in the length of the terminal pencil ; the foot is 0.72 instead ot 

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

 figures tiiat 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 of gapperi does not differ very 

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

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

 al)i)ut as broad as the rostral portion of the skull. The molar series is one-fifth 

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

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

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

 ccpiuls or is even less than the distance from the same point to the i)ack 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 

 oiic-tliird of the length. The interparielal 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 

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

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

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

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

 on top of the skull. 



