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Bryozoa
Amphiblestrum trifolium (Wood, 1844)
Nomenclature
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Family: CalloporidaeGenus: Amphiblestrum
- lectotype: B.M.(N.H.) Palaeont. Dept. B1642
- paralectotype: B.M.(N.H.) Palaeont. Dept. D6829
- paralectotype: B.M.(N.H.) Palaeont. Dept. D37772
- paralectotype: B.M.(N.H.) Palaeont. Dept. D6830
SUMMARY
Colonies are multiserial, often forming extensive encrusting sheets. The ancestrula is ovoidal with a longitudinally elliptical opesia, about 0.15 by 0.13 mm in diameter, and 5-6 circumopesial spine bases in its proximal half. Three zooids are budded from the ancestrula; these have opesia transitional in morphology between the elliptical opesia of the ancestrula and the trifoliate opesia characteristic of later astogenetic stages. Growing edges reveal the presence of a distal pore chamber with a large, ovoidal, frontally facing window, and a pair of smaller, lens-shaped pore chamber windows on each of the distolateral walls of the zooids.
Autozooids are about 0.40 mm long by 0.24-0.32 mm wide, typically elongate, rounded rhomboidal in outline shape. The frontal wall is an extensive, slightly convex, granular cryptocyst occupying a little more than one-half of the frontal area of the zooid. The opesia is large, trifoliate, about 0.16-0.22 mm in length and width, with an arched proximal edge and a pair of lateral indentations, one of which sometimes bears a spine base. Closure plates not infrequently occlude the opesia. Ovicells are abundant, prominent, globular, with a large, arch-shaped proximal window in the ectooecium exposing the entooecium.
Avicularia are abundant. Non-ovicellate autozooids are followed by a moderately large interzooidal avicularium directed proximally. This has a subrounded slightly inclined rostrum, ovoidal opesia and large area of gymnocyst in the part of the avicularium that is proximal with respect to the colony as a whole. Very occasionally, the single avicularium is replaced by two smaller avicularia side-by-side. Ovicellate autozooids are usually followed by a pair of avicularia at the distolateral corners of the ovicell which overlap the ectooecium of the ovicell; these are smaller than the avicularia following non-ovicellate zooids and are usually oriented distolaterally, although their orientation can vary.