MAMMALS OF NORTHERN COLOMBIA—HERSHKOVITZ 133 
Characters (of the species).—Upperparts from buffy, almost grayish, 
to tawny mixed with black, the median dorsal area when conspicuously 
darker, not sharply defined as a black band; spines stiffer, less tendent, 
and generally more prominent in middorsal region than in canicollis. 
Underparts white or with a drab to pale brown lateral line, which 
may extend midventrally as in canicollis. Upper surface of fore and 
hind feet with whitish to dark brown hairs. ‘Tail sparsely haired. 
Point of palatal notch on a level with or behind plane across middle of 
last molars. Enamel pattern of molariform teeth as described. 
Comparisons.—Cranial characters other than those cited are not 
diagnostic except in comparisons with species occurring in the same or 
nearby localities. Comparisons have already been made between 
P. canicollis and the form of guyannensis found in northern Colombia. 
Further comparisons are made in the descriptions of the other species. 
Remarks.—If we accept provisionally as subspecies of guyannensis 
all the forms so treated by Ellerman as well as others listed below, 
the races of the composite species are divisible into two geographical 
groups on the basis of the dental patterns. The first group, which 
includes typical guyannensis, occupies northern South America in the 
areas east of the Rio Magdalena and north of the Orinoco. The ma- 
jority of the spiny rats here show the following formula for the enamel 
pattern of the molariform teeth: Se). 
Of the forms not seen and not assigned to group by Ellerman, the 
following belong here: poliopus, ochraceus, and oconneili. The occur- 
rence of the geographically restricted P. canicollis within the range of 
this group of guyannensis indicates a probably recent origin from it. 
Some eastern Brazilian spiny rats, including P. g. arescens Osgood, 
from Maranhao, conform to the characters of this group but are 
widely separated geographically. 
In Central America, western Colombia west of the Magdalena, 
western Ecuador, and in the Amazonian region the populations 
of P. guyannensis show, generally, the less simplified formulae 
3-3-3-3, 3-3-4423 
a eee 
The latter formula approaches that of the Quadruplicatus group, but 
present material does not indicate that it grades into it. The holo- 
type and an adult topotype of steerei are characterized by that dental 
pattern. A series of topotypes of calidior in the U. S. National 
Museum also show the pattern with four folds in the last two upper 
molars and the lower premolar. The type skull of semispinosus has 
four folds in only the last upper molar, as well as in the lower pre- 
molar. The very clear description given by Winge (1888, p. 86) 
3 All formulae given and all references made to enamel folds are based on the number of outer folds in 
the upper and the number of inner folds in the lower molariform teeth. 
