There is a specimen of T. m. muticus labeled as from Mobile, Alabama (MCZ 1596), for which I believe the locality datum is incorrect. It is a young turtle having a well-defined pattern on the carapace and is without doubt a representative of T. m. muticus. Mobile is in the large drainage basin, of the Tombigbee, Black Warrior, Coosa and Alabama rivers, which is between the Escambia and Pearl rivers.
Yarrow (1882:28) reported a specimen of Amyda mutica, USNM 11630, from Gainesville, Florida. This record was questioned by Cahn (1937:179), and has been disregarded by subsequent authors. Stejneger (1944:23) lists this specimen number with uncertainty from Mt. Carmel, Illinois. The exact geographic provenance of this specimen is seemingly unknown.
Habitat.—I have collected eggs of T. m. calvatus on sand banks of the Escambia River, Florida. The Escambia River has a sand-gravel bottom, extensive sandy banks, a moderately-rapid current, and is flanked by a thick riparian forest. It is approximately 80 feet wide with fallen trees and brush intermittently emergent along the shoreline. The sand bar-habitat along the Pearl River has been mentioned by Anderson (1958:212). All records thus far are from lotic habitats.
Comparisons.—Trionyx m. calvatus is most closely related to Trionyx m. muticus. Both subspecies have the following characteristics: (1) no enlarged tubercles on the anterior edge of the carapace, (2) no ridge projecting from the nasal septum, and (3) a smooth carapace in adult males. These characters distinguish these two subspecies from the several subspecies of T. spinifer, and, except for the smooth carapace in adult males, from T. ferox. Another feature of T. m. calvatus and T. m. muticus, not known to be definitive or diagnostic but noticed on occasion, is the pale orange cast, in life, of the dorsal surface of the carapace and soft parts of the body in young of these turtles.
The spotted pattern of juveniles of calvatus is easily distinguished from the pattern of muticus (small dots, streaks and dashes) figured by Agassiz (1857, vol. 2, pt. 3, pl. 6, fig. 6), Smith (1950:154, fig. 104), Conant (1938:192, pl. 21, fig. 1; 1958, pl. 11, opposite p. 94), and Cahn (1937:177, pl. 24C).
Unfortunately, the distinctive dorsal spotting in young calvatus becomes obscure or absent in some adults of both sexes. Spotting in large males is not so well-defined as in juveniles; it may be absent (TU 17306.3), or indicated by two obscure spots (KU 17117), but is usually evident, at least posteriorly. The spotted pattern is absent in large females, which have a pale, mottled and blotched pattern of lichen-like figures; dorsal spots are obscure in TU 17305 (length of plastron, 56 mm).
Two additional features are, so far as known, universal in calvatus; these are: (1) the absence of striping on the dorsal surface of the snout, and (2) the presence of thick, black borders of the postocular stripe in adult males. These features have also been observed in some specimens of muticus; their presence in muticus cannot be properly evaluated at this time, and is seemingly not due to individual variation. These two characters, however, coupled with the distinctive juvenile pattern of spots, serve, in combination, to distinguish calvatus from muticus.
Discussion.—The two populations are recognized as subspecies because: (1) there is close resemblance, (2) the diagnostic characters pertaining to pattern are few and superficial, and (3) the geographic ranges are allopatric, but juxtaposed. It is probable that muticus and calvatus would be capable of interbreeding if they were not spatially isolated. It should be pointed out, however, that there is no evidence of intergradation between muticus and calvatus in the lower Mississippi Valley as has been reported for the subspecies of T. spinifer (Conant and Goin, 1948), and that the degree of difference between calvatus and muticus is greater than that between some subspecies of T. spinifer.
Specimens examined.—All the localities listed below are plotted on the distribution map [(Fig. 1]). Only those specimens of T. muticus muticus are listed that serve to delimit the range of T. m. calvatus. Fortunately, the identification of the specimens of muticus is certain as all show the characteristic juvenile pattern, except the large female, TU 7543, from southeastern Louisiana. USNM 95133-34 (carapaces and plastrons only) and TU 17236 are females, which lack the diagnostic spotted pattern of calvatus; the former are referred to this subspecies on geographic grounds (Pearl River at Columbia, Mississippi). TU 17236, from the Amite River, is dubiously relegated to calvatus on the supposition that this river and others in the Lake Ponchartrain drainage will yield the characteristic juveniles.
Trionyx m. calvatus (33 specimens): TU 13473, 16682, 17301, 17302-.1, KU 47116 (skull only), Escambia River, 2 miles east, 1 mile north Century, Escambia Co., Florida; TU 17303-.4, 17304-.3, Pearl River, Varnado, Washington Par., Louisiana; TU 17306-.3, Pearl River, 9 miles south Monticello, Lawrence Co., Mississippi; TU 16956, KU 47117-19, USNM 7655, Pearl River, vicinity of Monticello, Lawrence Co., Mississippi; TU 17236?, Amite River, near Baton Rouge, Louisiana; TU 13795, Bogue Chitto River, Enon, Washington Par., Louisiana; TU 17305, no data, Louisiana; USNM 95133-34, Pearl River, Columbia, Marion Co., Mississippi; UI 31071, Pearl River, 14 miles northeast Jackson, Rankin Co., Mississippi; Uncatalogued, see page [523], Leaf River, 3 miles southeast New Augusta, Perry Co., Mississippi.