Fig. 21. Basicranial length and greatest width of alveolar surface of maxilla on 52 skulls of T. spinifer (open circles) and 11 of the agassizi-form (solid circles; half shaded circle represents holotype of agassizi). Most skulls of the agassizi-form that exceed 43 mm. in basicranial length have a more expanded, alveolar surface of the maxilla than skulls of spinifer of approximately the same size. All skulls exceeding 50 mm. are those of females.
Some skulls of soft-shelled turtles from streams of the Atlantic Coast drainage, including the skull of the holotype of Platypeltis (= Trionyx) agassizi Baur (MCZ 37172, [Pl. 54]), show at least two differences from other skulls of asper and from those of other subspecies of T. spinifer. [Figure 20] shows that skulls of agassizi tend to have slightly smaller internal choanae (ratio IC/MB) than those of T. spinifer and T. ferox; there is seemingly little difference between skulls of ferox and spinifer, and little, if any, ontogenetic variation. [Figure 21] shows that most skulls of the agassizi-form that exceed 43.0 millimeters have a more expanded, alveolar surface of the maxilla than skulls of spinifer of approximately the same size; most skulls exceeding a basicranial length of 43.0 millimeters, and certainly all skulls exceeding 50.0 millimeters are those of females. Stejneger (1944:Pl. 30) also has provided photographs of a skull of the agassizi-form. It is of interest that of the 12 agassizi-form skulls (MCZ 37172; USNM 8708, 029034, 51981, 66859, 71681, 91282, 91310-11, 92521, 92583-84) that I examined some resemble ferox (Neill, 1951:9) in having the alveolar surfaces of the jaws broadened, and the greatest width at the level of the quadratojugal ([Table 3], Plate 54); also, the localities of all 12 skulls are within the geographic range of ferox. Skulls of ferox, however, have conspicuously broadened alveolar surfaces of the jaws only when they exceed in length the largest skulls of agassizi. The differences of skulls of the agassizi-form possibly reflect isolation in the Atlantic Coast drainage, and an adaptation in feeding habits. So far as I can ascertain, individuals occurring in rivers of the Atlantic Coast drainage in Georgia and South Carolina (referable to agassizi) do not differ consistently in external characters from individuals of T. s. asper that occur westward in the Apalachicola drainage.
Comparisons.—Trionyx s. asper can be distinguished from all other subspecies of T. spinifer by usually having more than one black line paralleling [506] the rear margin of the carapace. This character and the frequent fusion of the postlabial and postocular stripes on the side of the head distinguish asper from spinifer and hartwegi. T. s. asper differs from pallidus, guadalupensis and emoryi in having blackish spots and ocelli on the carapace, and lacking whitish dots or tubercles. T. s. asper resembles spinifer, hartwegi and pallidus but differs from guadalupensis and emoryi in having conical tubercles along the anterior edge of the carapace in large females. For additional differences see accounts of other subspecies.
Of the subspecies of T. spinifer, asper has a proportionately wide head that is closely approached in the subspecies guadalupensis and emoryi; T. s. asper differs from guadalupensis and emoryi in having a wider carapace, and resembles hartwegi and spinifer, but differs from the other subspecies in having the carapace widest at a plane approximately one-half way back on the carapace. T. s. asper differs from the other subspecies in having the shortest plastron.
Remarks.—Stejneger (1944:72-74) has discussed the history of Baur's Platypeltis agassizi. Briefly, Agassiz's description of Platypeltis ferox wherein he (1857:402) states that "The young ferox [Pl. 6, fig. 3] has two or three concentric black lines separating the pale margin …," was applicable to T. s. asper. Agassiz mentioned also that the young of his Aspidonectes asper (op. cit.:406) "as in Platypeltis ferox, … has … two or three black lines separating the pale rim of the posterior margin, …"; however, A. asper was distinguished chiefly by the "… prominent warts of the bony plates (loc. cit.)." Because the description of the pattern of ferox resembled that of asper, the validity of asper was not agreed upon by all workers. Boulenger (1889:245, footnote 1) referred to asper as a species that required "… further investigation."
Baur (1888:1121) realized that Agassiz's description of ferox was not that of Testudo ferox Schneider, and regarded the description of Agassiz as applying to a new species, which he named Platypeltis agassizii; Baur (op. cit.:1122) also recognized asper, referring it to the genus Aspidonectes. Baur designated a specimen from Georgia (the only individual seen by him) as the type of agassizi (Stejneger, op. cit.:73, footnote); this specimen is now MCZ 37172. Five years later (1893:218), Baur discussed generic relationships of trionychids, seemingly only on the basis of skulls (holotype of agassizi not mentioned), and referred agassizi to the resurrected genus Pelodiscus Fitzinger, 1835, which was distinguished from the other two American genera that Baur recognized (Platypeltis and Amyda) by having the "Posterior nares reduced in size by the inner and posterior extension of the maxillaries." Baur also transferred asper to the genus Platypeltis, and restricted the type locality of that species to "Lake Concordia, La." (op. cit.:220); the type locality of agassizi was restricted to "Western Georgia" (loc. cit.).
The name-combination, Pelodiscus agassizi, was not generally accepted. Hay (1892:144) and Siebenrock (1924:188) referred agassizi to the genus Trionyx. Hay regarded agassizi as a full species (see discussion by Stejneger, 1944:73), whereas Siebenrock considered it a subspecies of spiniferus; both authors regarded asper as a synonym of agassizi. Neither asper nor agassizi was mentioned in the first three editions of the Check List of North American Amphibians and Reptiles (Stejneger and Barbour, 1917, 1923, 1933); the [507] same authors in the fourth (1939:171, 172) and fifth editions (1943:212, 213) listed agassizi as a full species, and asper as a subspecies of spinifera. Stejneger (1944) used the same arrangement as set forth in the fourth and fifth editions of the Check List, and distinguished agassizi on the basis of cranial characters, namely, the small size of the internal choanae, the greater width of the alveolar surface of the lower jaw, and the position of the suture between the palatine and basisphenoid relative to the posterior edge of the temporal fossa. Neill (1951:9) regarded the peculiarities of the agassizi-type skull as inconstant, but recognized agassizi (and asper) as a subspecies of ferox. Crenshaw and Hopkins (1955) showed that asper did not intergrade with ferox. Schwartz showed that agassizi did not intergrade with ferox, and regarded agassizi as a synonym of T. s. asper (1956:17), but stated that agassizi possessed "wider crushing surfaces on the maxillae than does T. s. asper, even when skulls of the same size and sex are compared" (op. cit.:9).
The holotype of Platypeltis agassizi (MCZ 37172) is a dried adult female consisting of shell, skull and limb bones; the carapace is approximately 300 millimeters long (Schwartz, loc. cit.). I have examined only the skull of MCZ 37172 (Plate 54), and it is the largest of 12 agassizi-type skulls I have seen. The basicranial length is 72.5 millimeters, and the greatest width, which occurs at the level of the quadratojugals, is 52.9 millimeters. The agassizi-type skulls have been discussed under the subsection on variation.
The type locality of T. s. asper, Lake Concordia, Louisiana (lower Mississippi River drainage) as restricted by Baur (1893:220), is in an area of intergradation of three subspecies of Trionyx spinifer where most individuals are not typical of asper. The syntypes, the designation of MCZ 1597 as a lectotype, and Pearl River, Columbus, Marion County, Mississippi, as the type locality have been discussed elsewhere (Webb, 1960).