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Their common name refers to the triggering mechanism in the first two of their
three dorsal fin spines. The first spine can be erected by the fish and locked in
place by the second, the trigger, which must be withdrawn before the first spine is
There is a sharp spine on each gill cover; these spines, like those of the first
dorsal fin, are associated with venom glands and can produce very painful
wounds. Three species of weevers are found in the Old World, and one in the
New World, ...
Xenacanthus survived from the end of the Devonian Period, 345,000,000 years
ago, to about the end of the Triassic Period, 190,000,000 years ago.
Xenacanthus had a slim, elongated body with a low dorsal fin that extended
down most of it, ...
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This is the 198215th Edition, in two halves, with the Macropaedia and the Micropaedia. First published in 1768, the publisher is the oldest English-language encyclopedia still in print. Two Scottish ... Read full review