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 [488] 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.
T. spinifer is intermediate in size between ferox (larger) and muticus (smaller); the maximum size of the plastron in adult males is approximately 16.0 centimeters (14.0, muticus; 26.0, ferox), and of females, 31.0 centimeters (21.5, muticus; 32.5, ferox). The head for all subspecies of spinifer is proportionately narrower than in ferox but wider than in muticus.
In the skull, spinifer more closely resembles ferox than muticus, but differs from both ferox and muticus in usually having a well-developed opisthotic-exoccipital spur. Skulls of spinifer resemble those of muticus but differ from those of ferox in being widest at the level of the squamosal. Skulls of spinifer resemble those of ferox but differ from those of muticus in having the 1) ventral surface of the supraoccipital spine narrow proximally, and usually having a medial ridge, 2) foramen magnum rhomboidal, 3) distal part of opisthotic wing tapered, 4) lateral condyle of articular surface of quadrate not tapered posteriorly, and larger than medial articular surface, and 5) maxillaries in contact above premaxillaries. T. spinifer resembles ferox but differs from muticus in having the epiplastron bent at an approximate right angle. T. spinifer differs from ferox in having an epiplastral callosity, and from muticus in that the callosity does not cover the entire surface of the epiplastron. The hyo-hypoplastral suture is present more often in spinifer and muticus than in ferox.
Remarks.—Gray (1869:221) proposed the generic name Callinia as a new name for Aspidonectes as understood by Agassiz (1857:403). Gray referred Trionyx spiciferus (= spiniferus) Lesueur to Callinia. Stejneger (1907:514) designated Trionyx spiniferus Lesueur as the type species of Callinia. If Trionyx spiniferus Lesueur is considered to be generically distinct from other soft-shelled turtles, Callinia Gray, 1869, is available as a generic name with Trionyx spiniferus Lesueur, 1827, as the type species by subsequent designation.
Geographic variation.—T. spinifer is the most variable and widespread species of the genus in North America. Size of ocelli on the carapace decreases from east to west on turtles inhabiting waterways of the Upper Mississippi River drainage. The most impressive gradient, geographically oriented from western Louisiana to southwestern Texas is seen in each of several features: decrease in size of tubercles on the anterior edge of the carapace, reduction in contrast of pattern on the dorsal surface of limbs and side of head, change in pattern on the dorsal surface of the snout, and increase in the size of white spots on the carapace. But the gradient in size of white spots is reversed in T. s. emoryi, which has small white spots on the carapace. Some of the characters at the western terminus of this geographical gradient are shared with T. ater and muticus. Those subspecies comprising the emoryi group also show proportional characters that correspond closely with those of T. ferox.
On the basis of tuberculation and pattern on carapace, side of head, dorsal surface of limbs and snout, Trionyx spinifer may be divided into six subspecies.
Trionyx spinifer spinifer Lesueur
Eastern Spiny Softshell
Trionyx spiniferus Lesueur, Mém. Mus. Hist. Nat. Paris, 15:258, pl. 6, December, 1827.
T[rionyx] s[pinifer] spinifer Schwartz, Charleston Mus. Leaflet, No. 26:11, May, 1956.