=========================+==============+===============+==============
|_G. arenarius_|_G. personatus_|_G. tropicalis_
-------------------------+--------------+---------------+--------------
Zygomatic arches | parallel | narrower | narrower
| | posteriorly | posteriorly
Sagittal crest | absent | present | small
Squamosal knob | present | absent | present
Interparietal | subquadrant | triangular | triangular
Mesopterygoid fossa | V-shaped | U-shaped | V-shaped
Ratio, zygomatic breadth | | |
to basal length | 63.7-66.6 | 66.3-67.2 | 60.8-66.2
Ratio, mastoid breadth | | |
to basal length | 58.0-60.4 | 59.8-63.1 | 58.0-59.6
Border of premaxilla at | | |
incisive foramina | wedge-shaped | subquadrate | subquadrate
-------------------------+--------------+---------------+--------------
As can be seen in the accompanying table _tropicalis_ resembles
_arenarius_ in half of the eight characters considered, especially in
the presence of a knob on the zygomatic process of the squamosal (the
diagnostic character of _arenarius_ according to Merriam, 1895:140) and
in the shape of the mesopterygoid fossa. _G. tropicalis_ differs from
_arenarius_ principally in having a low sagittal crest in adult males
(lacking in _arenarius_) and in the shape of the interparietal bone,
which in _tropicalis_ is small (in some skulls difficult to see) and
triangular instead of being relatively large and subquadrate as in
_arenarius_.
_G. tropicalis_ resembles _personatus_ in half of the characters
considered, notably in shape of the interparietal bone, outline of
zygomatic arches, and constriction of the premaxillae where they border
the incisive foramina.
Considering the distinctive combination of characters possessed by
_tropicalis_, and its isolated, restricted geographic range (the
nearest known record of _Geomys_ is approximately 165 miles to the
north), _tropicalis_ is here regarded as a full species. A skull alone
examined from 10 miles northwest of Tampico does not differ from those
of other specimens studied.
The average weight of five non-pregnant July-taken females was 189.4
(180-200) grams. Weights of three males were 280, 270, and 255 grams.
Females are in all measurements smaller than males.
_Measurements._--Average and extreme measurements of five
females and three males from one mile south of Altamira are,
respectively, as follows: 243.5 (235-250), 260, 260, 265;
82.0 (78-85), 87, 93, 89; 32.2 (31-33), 35, 35, 33; ear from
notch in both sexes, 5; condylobasal length, 42.3
(41.3-43.1), 46.0, 48.0, 46.2; zygomatic breadth, 26.6
(25.1-27.7), 30.4, 31.2, 30.5; interorbital constriction,
6.2 (6.1-6.3), 6.0, 6.2, 6.3; length of nasals, 14.6
(14.0-15.3), 17.0, 16.8, 15.9; alveolar length of maxillary
tooth-row, 9.0 (8.6-9.3), 9.9, 10.0, 9.4.
_Records of occurrence._--Specimens examined, 19: 1 mi. S
Altamira, 18; 10 mi. NW Tampico, 1.
Additional record: Altamira (Goldman, 1915:134).
=Heterogeomys hispidus negatus= Goodwin