410


Journal of Paleontology 91(3):407–416


1'. Septa arranged in multiples of 20, or 27–32 2. Septa arranged in five cycles (96 septa)


Table 2. Key to the species of Periplacotrochus. 1. Septa hexamerally arranged in four or five cycles


2'. Septa arranged in four (or fewer) cycles (40–48 septa) 3. Corallum regularly curved (cornute)


3'. Corallum not cornute, but may be irregularly bent


4. GCD:LCD = 1.5–1.67 (calice outline elliptical in cross section) 4'. GCD:LCD = 1.02–1.115 (calice almost circular) 5. Corallum with a full fourth cycle of septa (48) 5'. Corallum lacking lateral pairs of S4 (40 septa)


6. Septa arranged in three size classes, including 20 primary septa (80 septa) 6'. Septa arranged in three size classes, including 27–32 primary septa (104–128 total septa)


P. corniculatus 4


P. deltoideus 3


2 6


P. elongatus 5


P. pueblensis P. inflectus P. cudmorei P. magnus


Transverse division lacking; corallum fixed to fragment of substrate by small pedicel elliptical in cross section; pedicel not stereome reinforced. Pali and paliform lobes absent; lamellar columella prominent. Calicular edge smooth; septa often slightly exsert.


Occurrence.—Late Eocene to middle Miocene: Victoria, South Australia, Tasmania.


Etymology.—The Greek prefix peri, meaning near, prefaces Placotrochus to show its close relationship to that genus. Gender: masculine.


Remarks.—Periplacotrochus differs from Flabellum in having a lamellar columella, and from Placotrochus by being pedicellate (not transversely dividing). Key to the species of Periplacotrochus is shown in Table 2.


Periplacotrochus deltoideus (Duncan, 1864) new combination Figure 2.1–2.4


1864 Placotrochus deltoideus Duncan, p. 164, 167, pl. 5, fig. 5a–c.


1870 Placotrochus deltoideus; Duncan, p. 300, 309, 311, pl. 20, fig. 4a, b.


1875 Placotrochus deltoideus; Tenison-Woods, p. 17. 1876 Placotrochus deltoideus; Duncan, p. 341. 1878b Placotrochus deltoideus: Tenison-Woods, p. 315. 1878c Placotrochus deltoideus; Tenison-Woods, p. 65. 1927 Placotrochus deltoideus; Felix, p. 417. 1989b Placotrochus deltoideus; Cairns, p. 75, pl. 40, fig. a–c.


Undated Placotrochus deltoideus; Fitzgerald and Schmidt, p. 3 (color illustration).


Types.—Five syntypes are deposited at the NHMUK (PI R29269-29273). Type locality: Muddy Creek, Victoria (Balcombian, middle Miocene).


Diagnosis.—Corallum with five cycles (96 septa) of hexam- erally arranged septa.


Occurrence.—Janjukian (= Rupelian, late Oligocene): Torquay, Victoria. Balcombian (= Langhian, middle Miocene): Muddy Creek, Balcombe’s Bay, and Beaumaris, Victoria. Miocene: northwestern Tasmania.


Description.—The corallum has a bimodal edge angle, the lower thecal edges making an angle of 71º–87º, the upper thecal edges diverging at only 48º–60º (Fig. 2.1); the lower thecal edges are acute and usually bear a low, discontinuous crest (variety bursarius of Duncan, 1864). The thecal faces are planar (not convex), having a face angle of 30º–39º. The largest corallum (USNM 67953, Fig. 2.1–2.4) is 27.2mmin height and 27.2 × 19.3mmin calicular diameter. The calice is elliptical, the GCD:LCD ranging from 1.4 to 1.7; the H:GCD ranges from 1.0 to 1.1. The greater axis of the small (1.5–1.6 × 0.95–1.0 mm), elongate pedicel is aligned with the GCD. Each thecal face is ribbed with low C1–3 (Fig. 2.1). Above a GCD of 20mmthe septa are consistently arranged


in five complete cycles (Fig. 2.3–2.4): S1–3>S4>S5 (96 septa). Below that diameter, coralla have only 20–22 major septa (not 24) and a corresponding lesser number of higher cycle septa. The S1–3 have slightly sinuous axial edges, the lowermost edges fusing with the columella. The S4 are about half the width of the S1–3, and the S5 are rudimentary, but consistently present. The fossa is of moderate depth and contains a prominent lamellar columella, consisting of a single thick lamella.


Materials.—Muddy Creek (Clifton Bank), Victoria, middle Miocene, one corallum, USNM 67955 (topotypic). USGS 10674, Muddy Creek, Victoria, middle Miocene, eight coralla, USNM M353589 (topotypic). Balcombe’s Bay, Victoria, Balcombian (middle Miocene), five coralla, USNM 67953 and USNM M353578. USGS 10809, Mornington, Balcombe’s Bay, Victoria, middle Miocene, eight coralla, USNM 77072 and USNM M353921. USGS 10808, Torquay, Victoria, Janjukian (late Oligocene), six coralla, USNM M353590.


Remarks.—Periplacotrochus deltoideus is very similar to P. magnus, resembling a smaller version of that species. However, P. deltoideus can be distinguished by having a


Figure 2. (1–4) Periplacotrochus deltoideus, USNM 67953, Balcombe’s Bay, Victoria, middle Miocene, lateral, edge, calicular, and oblique calicular views, respectively. (5–7) P. corniculatus, holotype, NMV P27069, lateral, edge, and calicular views, respectively. (8–12) P. elongatus, USNM 10674, Muddy Creek, Victoria, middle Miocene: (8) specimen showing lamellar columella; (9–12) lateral, edge , calicular, and oblique calicular views of same specimen. (13–15) P. pueblensis, holotype, NMV P27106, lateral, edge, and calicular views, respectively. (1–6, 8–14) Scale bars = 10mm; (7, 15) scale bars = 5mm.


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