1012


Journal of Paleontology 91(5):1001–1024


2000 Plagiochasma cruciferum; Smith and Jeffery, p. 163, text-fig. 66D–H.


2001 Plagiochasma cruciferum; Kroh, p. 392, figs. 12–14, pl. 3, figs. 4–8.


Type.—Holotype ANSP 1464, from the Vincentown Formation, Timber Creek, New Jersey.


Description.—Small, outline oval to elongate, tall, rounded profile with inflated oral surface. Apical disc semi-ethmolytic, slightly eccentric anteriorly, four genital plates, each penetrated by large pore (Fig. 8.4, 8.5). Madreporite well developed, expanding to separate posterior genital plates. Ocular plates distinct, posterior pair in contact. Ambulacra narrow, aborally subpetaloid, anterior pair diverges at 140º, posterior at 60º; pores round, nearly equal size, becoming unequal and strongly oblique below the margin, pore pairs weakly conjugate. Subpetaloid ambulacral plates simple (Fig. 8.1), but becoming occluded near the peristome in the pyrinoid fashion (Fig. 8.2, 8.3). Periproct large, longitudinal, often distinctly pointed adapically, marginal in larger specimens,


strongly supramarginal in smaller, more juvenile specimens, with slight indentation of posterior margin, but without a clearly defined groove. Peristome oblique, central to slightly anterior, sunken, proximal plates rotated nearly vertical to oral plane, no bourrelets, greatly reduced phyllode, no buccal pores, no sphaeridial pits, pore pairs crowded into offset triads. Single narrow basicoronal plates, deeply inset and generally only visible internally, abut peristome in all interambulacral columns. No evidence of lantern supports, either auricles or apophyses, although no juveniles were examined internally. Tubercles small, evenly distributed over aboral and oral surfaces, perforate, sunken.


Additional material.—MMNS 4889, Clayton Formation, Randolph County, Georgia; MMNS 5300, MMNS 7173, MMNS 8245, and MMNS 8246, from the Clayton Formation, Butler and Lowndes Counties, Alabama.


Occurrence.—Vincentown Formation, Timber Creek, Gloucester Co., New Jersey (exact locality unknown). Clayton Formation (McBryde Member), Resource Management Services Palmyra


Figure 8. Plagiochasma cruciferum (Morton, 1830) from the Clayton Formation, Palmyra tract, Lowndes County, AL: (1) MMNS 7173.1, aboral plate pattern; (2) MMNS 7173.1, detail of ambulacrum III, oral, from anterior edge of peristome to anterior margin; (3) MMNS 5300.4, oral plate pattern; (4) MMNS 8245, detail of semi-ethmolytic apical disc; (5) MMNS 8246, detail of semi-ethmolytic apical disc. Scale bars are (1, 3) 10mm; (2) 2mm; (4, 5) 1mm.


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