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Head and Soft Parts.- Head wide in comparison to other members of the C. longicollis group (head width at tympanum/CL = 0.15 to 0.18; mean = 0.17; N = 8), with small irregular scales covering the postorbital regions, but with a smooth skull roof area. Rostrum blunt, not protruding or beak-like. Neck covered with prominent, bluntly pointed tubercles with wide bases. Soft parts gray to brown dorsally, light yellow to cream ventrally, but with the head, neck, and limbs variably suffused with red or pink. Head and neck of hatchlings and juveniles marked with orange to red to dark cherry colouring. Iris usually dark brown, with the inner pupillary margin very light in colour.
Male with tail thicker at base and extending noticeably beyond carapace rim in comparison to that of females; adult males smaller (approx. 72%) in size than adult females.
Osteology
Skull.- The skull of C. canni (n=9) (Fig. 2) is robust anteriorly with a broad rhamphotheca, highly emarginated both from below and behind, to the extent that the parietal arch has disappeared as is typical of the Chelodina. Only a single frontal bone is present which posteriorly partially divides the parietals, a feature unique to this species. The dorsal ridge of the parietals immediately narrows to a ridge, similar to that of C. reimanni (n=6) but differentiating it from C. novaeguineae (n=8). The anterior edge of the skull is rounded when viewed from above, which differs from C. novaeguineae which is narrowed and angular. A major difference between C. canni and other members of the C. novaeguineae complex (C. novaeguineae, C. reimanni and C. mccordi) is the presence of paired premaxilla in C. canni, a feature previously noted by Rhodin (1994a).
There is contact between the vomer and the pterygoids and the vomer is narrowed posteriorly, partially dividing the anterior half of the pterygoids, whereas in C. novaeguineae the vomer is widened and angular posteriorly, which widely separates the anterior of the pterygoids. The canalis caracoticus internus is completely open with only the anterior foramen present (foramen canalis caracoti internus). This condition contrasts to that of C. novaeguineae where the canalis caracoticus internus are partially closed forming a channel within the prootic. Like all other members of the C. longicollis group (although variably present in rudimentary form in C. longicollis), C. canni has no foramen retropterygoideum (chelid foramen; McDowell, 1983; Bour and Pauler, 1987; Rhodin, 1994a).
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The crista supraoccipitalis is small in this species and is markedly narrower than the ridge of the parietals, wider in C. novaeguineae. The cristae paroccipitalis are large and relatively narrow in C. canni and extend horizontally from the base of the occipital condyle; this unit is smaller in C. novaeguineae and not as visible.
Shell.- The shell of Chelodina canni (Fig. 3) is typical of members of the C. longicollis group in that the pleural bones are all thin and narrow, with only minor antero-posterior lengthening of the first pleural. The typical chelid compliment of 8 pairs of pleurals, 11 pairs of marginals, a suprapygal, pygal and nuchal bone are present. Neural bones are not exposed and do not form a contiguous series, the typical Australasian Chelid condition (Rhodin and Mittermeier, 1977; Thomson and Georges, 1996). The nuchal bone in C. novaeguineae is narrower posteriorly and more deeply divides the anterior edges of the first pleural pair than that seen in C. canni. However, this condition would appear to be affected by the carapace width to length ratio in a range of chelids.
Long-necked turtles of the C. novaeguineae complex (here defined to include C. novaeguineae, C. canni, C. reimanni and C. mccordi), within the C. longicollis group, all share the feature of an enlarged anterior bridge strut (Thomson et al., 1997b; Thomson and Mackness, 1999; Thomson, 2000). Chelodina novaeguineae has the suture for the anterior bridge strut flush against the ventral surface of the carapace, however, C. canni has this suture raised up from the carapace on a ridge formed from the first pleural. This feature is shared with C. reimanni and C. mccordi. There is a medial constriction in the anterior bridge strut suture in C. novaeguineae that is present but not as obvious in C. canni. The anterior bridge strut is not present in the other members of the C. longicollis group.
The pelvis of the C. longicollis group species sutures to the seventh and eighth pleurals and to the suprapygal bone. In C. novaeguineae (n=8) the medial edge of this suture is parallel to the vertebral column and hence the rib of thoracic vertebra 9 is equal in length to that of vertebra 10. In C. canni (n=9), however, the medial edge of the suture is closest to the vertebrae adjacent to thoracic 10, hence, the rib of thoracic 9 is approximately twice as long as that of thoracic 10.
The plastron of this species, apart from being wider than most others (except C. longicollis) is little different in its osteology from other C. longicollis group species.
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