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(Sars, 1866)

Description:
Valves with reticulate ornament modified by the presence of five or six tubercles; the subcentral, posterodorsal and posteroventral tubercles are the most prominent, while weaker ones may be seen posteriorly, anterodorsally and anteroventrally. Male copulatory appendage with a long, tapering distal process directed anteriorly and two smaller processes directed ventrally and posteriorly.

Habitat:
A sublittoral species found at depths down to about 75 metres.

Remarks:
Horne (1983b) has described and illustrated the considerable variation in outline and ornament shown by this species throughout its extensive geographical range. British specimens, like those from Norway, have less conspicuous tuberculation and secondary reticulation, and are also smaller, than those from the Arctic.

Distribution:
Most abundant in Arctic waters; in Britain, its present-day distribution appears to be confined to Scotland and eastern England as far south as the Thames Estuary.

Robertsonites tuberculatus