. Bulletins of American paleontology. Ordovician-Silurian Colonial Corals: Young and Ellas 75 'Vc ^^ tr^ 1 jgs^S^ K^ ^ Ql ^ sir /x W A mm. 1 mm Text-figure 21. —A-D, Serial transverse sections illustrating corallite increase in a corallum of Catenipora lawrencica: USNM 485780b (holotype). Section-interval 23-2 (Lawrence Quarry) (see also PI. 12, figs. 3-6). Corallite walls and septa are solid, tabulae are finely stippled; a rank of Halysites atexandricus is coarsely stippled, a possible corallite of Aulopora is hachured, br = brachiopod shell; scale bar is shown. Heights in mm are above an
. Bulletins of American paleontology. Ordovician-Silurian Colonial Corals: Young and Ellas 75 'Vc ^^ tr^ 1 jgs^S^ K^ ^ Ql ^ sir /x W A mm. 1 mm Text-figure 21. —A-D, Serial transverse sections illustrating corallite increase in a corallum of Catenipora lawrencica: USNM 485780b (holotype). Section-interval 23-2 (Lawrence Quarry) (see also PI. 12, figs. 3-6). Corallite walls and septa are solid, tabulae are finely stippled; a rank of Halysites atexandricus is coarsely stippled, a possible corallite of Aulopora is hachured, br = brachiopod shell; scale bar is shown. Heights in mm are above an arbitrary datum within a mature part of the corallum; numbers in italics represent individual corallites; an asterisk indicates the first appearance of a particular corallite. USNM 485780b). Other specimens are used for com- parison. The protocoralHte is not known. The lowest known stages of corallum growth have thick-walled, septate corallites that are not distinguishable from those oc- curring higher up. Ranks in the lower part of the cor- allum are short and straight or slightly curved, com- monly composed of one to three corallites. They are initially spreading and do not join to form lacunae. Higher in the corallum, ranks are subparallel, longer and more sinuous, commonly having two to four cor- allites. Ranks join to surround subpolygonal lacunae that become more elongate with height. The shape of ranks may vary depending on whether lateral growth was blocked by large skeletal grains (Text-fig. 21). In longitudinal section, some corallites extend through much of the height of the corallum. Corallite increase may be interstitial (between adult corallites in a rank) or lateral (at the end or side of a rank). Interstitial increase apparently occurred through fission of the intercorallite wall. New corallites pro- duced by interstitial increase are separated by thin walls from the adult corallites on either side (Text-fig. 21B, C, note corallites 10, 11, 16). In transverse
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