MAMMALS OF NORTHERN COLOMBIA—HERSHKOVITZ 131 
cation of the enamel pattern of the molariform teeth. Given the 
‘normal’ number of folds for each species, as outlined in the key, and 
beyond, the greater tendency is for the elimination of one of the 
‘normal” number of folds than for the reappearance of an additional 
fold. 
A satisfactory determination can be made of the “normal” pattern 
and the significant deviations therefrom only in a large series that 
includes individuals with unworn and fully formed but unerupted 
teeth. With wear, an enamel fold becomes isolated from the margin 
of the crown and appears as an enamel island. Because of inequalities 
in the depth along its base the fold may break up into two islands. 
Furthermore, a sinuous curve in the original fold may result in one of 
the subsequently formed islands assuming a position such as to appear 
as a distinct enamel fold. Frequently any two adjacent folds may be 
united in the form of a single fold with two branches. With wear, one 
of the branches becomes isolated from the other, thus restoring, in 
appearance, the ‘“‘normal”’ pattern of the tooth. As a consequence of 
the order of dental succession, the premolar and first molar are con- 
siderably more worn, their patterns more modified and divergent from 
the ‘normal’ pattern than the remaining molars. Though many 
instances of mechanically induced types of variation may be cited, the 
foregoing formulae are sufficiently broad to facilitate the specific 
identity of most of the spiny rats of the genus. 
PROECHIMYS CANICOLLIS (Allen) 
Echimys canicollis Autumn, Bull. Amer. Mus. Nat. Hist., vol. 12, p. 200, 1899. 
Type locality —Bonda, near Santa Marta, northwestern base of the 
Sierra Nevada de Santa Marta, Department of Magdalena, Colombia. 
Distribution —Northern Colombia. Known from the Rio Cesar 
Valley and the northwestern and eastern bases of the Sierra Nevada 
de Santa Marta, Department of Magdalena, and from the Ciénaga de 
Gudjaro, about midway between Barranquilla and Cartagena, De- 
partment of Atl&ntico. 
Characters. —A pale spiny rat, upperparts buffy to ochraceous mixed 
with black, no well-defined median dorsal band present; spines soft 
and mostly hidden by the soft annulated hairs. Underparts with 
gray lateral lines, which may extend mesially over the whole ventral 
surface or, more commonly, over neck and throat only. Upper 
surface of fore and hind feet well covered with whitish hairs. Tail 
thickly covered with comparatively long hairs. Walls of mesoptery- 
goid fossa extremely fenestrated. Molariform teeth as described. 
Comparisons.—Distinguished externally from P. guyannensis mincae, 
the only other representative of the genus found in the same general 
region, by hairier tail, softer spines, and slightly shorter ear and hind 
