Homalopoma, Carpenter, 1864

Kiel, Steffen, Fernando, Allan Gil S., Magtoto, Clarence Y. & Kase, Tomoki, 2022, Mollusks from Miocene hydrocarbon-seep deposits in the Ilocos-Central Luzon Basin, Luzon Island, Philippines, Acta Palaeontologica Polonica 67 (4), pp. 917-947 : 936-937

Homalopoma ? sp.

Fig. 18F .

Material.— One specimen from block B ( NRM Mo 192583: W = 11.8 mm, H = 9.6 mm) .

Remarks.— Homalopoma has been reported from fossil seeps of Cretaceous age onwards. The specimen reported here has much finer and more numerous spiral cords compared to both Homalopoma abeshinaiensis Kaim, Jenkins, and Hikida, 2009 , from the Upper Cretaceous of Hokkaido, Japan ( Hikida et al. 2003; Kaim et al. 2009), and Homalopoma domeniconii Moroni, 1966 , from Middle–Late Miocene seep deposits in Italy ( Moroni 1966). Species with similar spiral ornament but taller shells and a more convex whorl profile include the middle to late Eocene Homalopoma ? sp. from the Humptulips seep deposit in Washington state, USA ( Goedert and Squires 1990), Homalopoma wattsi ( Dickerson, 1916) from a middle Eocene wood fall in Washington (Kiel 2008), Homalopoma sp. from fossil seeps of somewhat uncertain Cenozoic age in Barbados ( Gill et al. 2005), and the extant Homalopoma bicolor Okutani, 2001 , from the northern Mariana Ridge ( Okutani 2001). Homalopoma tosaense Habe, 1953 , reported from the Plio-Pleistocene of Luzon Island, Philippines ( Helwerda et al. 2014), is smaller, taller, and has very distinct spiral keels rather than fine spiral cords has the specimen reported here from the Northern Cement quarry.

Of very similar overall shape are species of Phanerolepida , including Phanerolepida rehderi MacNeil, 1960 , from the Plio-Pleistocene of Okinawa, southern Japan ( Noda 1988: pl. 5: 3) but the Philippine specimen illustrated here clearly has spiral sculpture instead of the net-like rhombohedral pattern of Phanerolepida ( Hickman 1972) .

Figures

Fig. 18. Limpets, neomphalids, and vetigastropods from Upper Miocene seep deposits of the Amlang Formation in Pangasinan province, Luzon, Philippines. A, B. Unidentified limpet gastropod. A. NRM Mo 192715, small specimen, mostly an internal mold. B. NRM Mo 192656, large specimen with sculpture. C. The neomphalin Retiskenea sp., NRM Mo 192540, apical view on specimen embedded in rock matrix. D, E. The seguenzoid Cataegis sp. D. NRM Mo 192666; note sculpture on D2. E. NRM Mo 192665, specimen showing the strong spiral ribs, in apical (E1) and abapertual (E2) views. F. The colloniid Homalopoma sp., NRM Mo 192583, views on entire shell (F1–F3) and close-up on spiral sculpture (F4).