=========================+==============+===============+============== |_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.