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Hokonuia limaeformis, left and right valves, showing the robust shell and distinctive 'twist' in the shape
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Hokonuia limaeformis
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Hokonuia limaeformis is a common, but far from abundant, bivalve in the Triassic rocks of the Nelson area. It is often found in association with the much more proliific bivalve Manticula problematica, in which case it is easily overlooked, because superficially the two species appear similar (see image, rihgt). Closer inspection, however, shows that M. problematica has a straight (or almost straight) beak and a more-or-less symmetrical outline - i.e. a line through the centre of the beak extends down the middle of the shell. In H. limaeformis in contrast, the beak is skewed to one side, typically by about 15 degrees, and downwards (i.e. towards the opposing shell) in an almost claw-like manner. The outline of the shell is also slightly asymmetrical. Compared to M. problematica, the shells tend to be less elongated (though rounded examples of M. problematica do occur).
Confusion is also possible with Oreta coxi. Distinction is possible, however, because the shells in H. limaeformis are both inflated and robist, while in O. coxi the left valve is relatively flat and the shell much thinner. Differences in the beak also help to tell the two apart. In O coxi, like M. problematica, it is relatively straight. The two species are also distinguished by their stratigraphic position. H. limaeformis occurs in beds of Otamitan to Warepan age; O. coxi is confined to the older Oretian stage of the Triassic. Trechmann (1917) provides a detailed description of the species (see panel below), and further descriptions and images are provided by Marwick (1953). |
Hokonuia limaeformis, showing the skewed beak and the deep 'folds' made by the growth-lines.
Hokonuia limaeformis (left) in association with Manticula problematica (right). Note the differences in the shape of the beaks.
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Description by Trechmann (1917):
Shell rather higher than long, hinge-line straight, measuring about two-thirds of the length of the shell. Beak rather pointed, anteriorly situated and directed forwards, not projecting above the hinge-line. There is a curved truncation in front of and below the beak of the right valve, which extends for about a third of the length of the anterior margin and is bounded posteriorly by a raised ridge, on and behind which three or four faint radial ridges occur. The tongue-like projection of the right valve is directed towards the opposite valve at about a right angle from the plane of the right valve. The posterior wing is angular and flattened. The hinder margin is gently curved, the anterior margin more rounded, and the lower margin is well rounded. The growth-lines are prominent and irregular, and the surface of the shell towards the lower margin tends to become wavy and irregular.
Shell rather higher than long, hinge-line straight, measuring about two-thirds of the length of the shell. Beak rather pointed, anteriorly situated and directed forwards, not projecting above the hinge-line. There is a curved truncation in front of and below the beak of the right valve, which extends for about a third of the length of the anterior margin and is bounded posteriorly by a raised ridge, on and behind which three or four faint radial ridges occur. The tongue-like projection of the right valve is directed towards the opposite valve at about a right angle from the plane of the right valve. The posterior wing is angular and flattened. The hinder margin is gently curved, the anterior margin more rounded, and the lower margin is well rounded. The growth-lines are prominent and irregular, and the surface of the shell towards the lower margin tends to become wavy and irregular.
Further Reading:
Marwick, J. 1953 Divisions and faunas of the Hokonui System. Paleontological Bulletin 21. Wellington: New Zealand Geological Survey.
Trechmann, C.T. 1917 The Trias of New Zealand. Quarterly Journal of the Geological Society 291, 165-246 (+ 10 plates).
Marwick, J. 1953 Divisions and faunas of the Hokonui System. Paleontological Bulletin 21. Wellington: New Zealand Geological Survey.
Trechmann, C.T. 1917 The Trias of New Zealand. Quarterly Journal of the Geological Society 291, 165-246 (+ 10 plates).