101 
Skull. The skull of P. laingi has the general characters of the P. parvus 
group, with greatly projecting nasals extending 3-5 mm. beyond plane of 
anterior edge of upper incisors; broad, flat interorbital region; cranium 
slightly arched, but greatly depressed in occipital region, sloping down- 
wards from frontal-parietal suture; lambdoidal ridge indistinct; incisive 
foramina much aborted, short and narrow, from 1*5 to 2 mm. long. 
Molars three in each side of upper and lower jaws, rather wide and short, 
and upper molars sloping backwards; premolars one on each side, the 
anterior column of upper premolars narrow and sloping sharply back- 
wards; upper incisors sharply curved backwards; mandible short with 
incisors sloping forward. Audital bullse large and rounded, nearly meeting 
at anterior ends; zygoma very slender, the jugals parallel with main axis 
of skull, sometimes slightly incurved in middle part; interparietals roughly 
pentagonal in shape, considerably wider than long. 
Skull Measurements. Type (male) : condylo-basal length of Hensel, 
20*0; occipito-nasal length, 27*5; mastoidal breadth, 14*5; least inter- 
orbital breadth, 5*0; interparietal length, 4*0; interparietal width, 5*0; 
length of nasals, 10*0; length of incisive foramina, 2*0; ratio of interparietal 
width to condylo-basal length, 25*0. Allotype (female): condylo-basal 
length, 19*5; occipito-nasal length, 26*2; mastoidal breadth, 6*5; inter- 
parietal length, 3*5; interparietal width, 5*0; length of nasals, 10*2; length 
of incisive foramina, 2*0; ratio of interparietal width to condylo-basal 
length, 26*3. For averages See table of comparative measurements. 
Comparisons. The skull of P. laingi is intermediate in size between 
P. lordi and P. parvus, with interparietals slightly wider than in lordi and 
with shallower notch made by occipital, narrower than in parvus, the ratio 
of interparietal width to condylo-basal length in laingi averaging 26*4 in 
males, and 26*3 in one female, the same ratio in lordi being 25* 3-25*5, and 
in parvus 28*6-31*4 in specimens examined and measured. Rostrum in 
laingi is somehwat shorter than in lordi; audital bullae large and rounded, 
slightly smaller than in lordi and larger than in parvus; incisive foramina 
very short and narrow, nearly obliterated. 
W. H. Osgood, in his key to species and subspecies 1 separates the 
forms of the lt parvus group” into two sections as follows: 
"Interparietal narrow, ratio of its width to basilar length of 
Hensel 2 about 25 lordi, columbianus. 
Interparietal wide, ratio of its width to basilar length of Hensel 
about 27 alticola, magruderensis, parvus, olivaceus, molli - 
pilosus,” 
Perognathus laingi has the above ratio about intermediate between 
the two sections of the " parvus group,” averaging slightly nearer the section 
including typical parvus, with at least one specimen having ratio as large 
as in parvus. When Osgood monographed Perognathus he examined 131 
specimens which he referred to P. lordi, 34 of them being from British 
^‘Revision of the Pocket Mice of the Genus Perognathus”: North American Fauna, No. 18, p. 14 (1900), 
*The basilar length of Hensel is measured from the anterior margin of the foramen magnum to the posterior 
rim of alveolus of the middle incisor. 
