_Diagnosis._--Juvenal pattern uniform tan or brownish lacking markings, having whitish dots or spots, or having well-defined, blackish ocelli or spots; surface of carapace "sandpapery" in adult males; conical projections (in some subspecies) along anterior edge of carapace in large females; contrasting pattern of blackish marks on pale background (in some subspecies) on dorsal surface of limbs of adult males. Opisthotic-exoccipital spur well-developed; epiplastral callosity, when present, not covering entire surface. _Description._--Septal ridges present; external and proportional characteristics variable (see accounts of subspecies); range in length of plastron (cm.) of ten largest specimens of each sex (mean follows extremes), males, 13.8-16.0, 14.4; females, 26.0-31.0, 28.0. [Illustration: FIG. 19. Geographic distribution of _Trionyx spinifer_. Guide to subspecies: 1. _T. s. spinifer_ 2. _T. s. hartwegi_ 3. _T. s. asper_ 4. _T. s. pallidus_ 5. _T. s. guadalupensis_ 6. _T. s. emoryi_ ] Greatest width of skull usually at level of squamosal (74%); foramen magnum rhomboidal; ventral surface of supraoccipital spine narrow proximally, usually having medial ridge; opisthotic-exoccipital spur well-developed (66%); distal part of opisthotic wing tapered, not visible in dorsal view; lateral condyle of articular surface of quadrate larger than medial articular surface, not tapered posteriorly; maxillaries in contact above premaxillaries (88%); usually a combination of seven neurals, seven pairs of pleurals and contact of seventh pair of pleurals (83%); angle of epiplastron approximately 90 degrees; callosities when present on epiplastron not covering entire surface; hyo-hypoplastral suture usually present. _Comparisons._--_Trionyx spinifer_ can be distinguished from _T. ferox_ and _T. muticus_ by the presence of any one of the characters mentioned in the "Diagnosis." Both sexes and all sizes of _T. spinifer_ resemble _ferox_ but differ from _muticus_ in having septal ridges. Most individuals of _T. spinifer_ (except some large females) resemble _muticus_ but differ from _ferox_ and large females of _ater_ in having a pale outer rim that is separated from the ground color of the carapace by a distinct (_spinifer_) or dusky (_muticus_) dark line. Large females of the subspecies _spinifer_, _hartwegi_, _asper_ and _pallidus_ may have enlarged conical projections along the anterior edge of the carapace and, unless these projections are considerably worn, are readily distinguished from large females of _ferox_ (flattened, knoblike prominences), and _muticus_ and _ater_ (smooth surface, no prominences). Large females of the subspecies _guadalupensis_ and _emoryi_ resemble _muticus_ and _ater_, and to some extent _ferox_, in having low, scarcely elevated prominences along the anterior edge of the carapace. Some females of _emoryi_ resemble _ferox_ in that the plastron extends farther forward than the carapace.