Cainozoic Research, 15(1-2), pp. 147-153, October 2015 147 New species of Neritidae (Gastropoda, Neritimorpha) from the Solent Group (late Eocene and early Oligocene) of the Hampshire Basin Malcolm Francis Symonds The Cottage in the Park, Ashtead Park, Ashtead, Surrey KT21 1LE, England; [email protected] Received 18 July 2015, revised version accepted 6 August 2015 Three previously undescribed species in the family Neritidae are introduced from the Solent Group of the Isle of Wight, Hampshire Basin, southern England. New species: Clithon (Pictoneritina) pococki sp. nov., Clithon (Pictoneritina) mortoni sp. nov. and Clithon (Pictoneritina) hillae sp. nov. The palaeoenvironments of the Solent Group Pictoneritina are compared with the habitats of Recent members of the subgenus. KEY WORDS: Neritimorpha, Neritidae, Clithon, Pictoneritina, new species, late Eocene, early Oligocene, Isle of Wight, palaeoecology Introduction surface, inner side with two apophyses connected by a calcareous callus. The taxonomy of the Neritidae of the Solent Group was reviewed by Symonds (2006), a new genus erected and Discussion – De Montfort did not specify the gender of two new species described. Since then more material Clithon and it has been treated by most authors as a mas- from the Solent Group has been discovered, mostly as a culine noun. However, in accordance with Article 30.2.4 result of continuing fieldwork on the Isle of Wight, and of the International Code of Zoological Nomenclature the primary purpose of this paper is to describe three (International Commission on Zoological Nomenclature, new species of Clithon which have been found there. The 1999), it should be regarded as neuter (Steve Tracey, ICZN palaeo ecology of Clithon (Pictoneritina) is compared with Secretariat, pers. comm.). the ecology of extant species of the subgenus and of Cli­ thon (Clithon). All specimens referred to are held in the Subgenus Pictoneritina Iredale (1936, p. 288) Department of Earth Sciences at the Natural History Mu- seum, London (Institution code: NHMUK). Type species – Neritina oualaniensis Lesson (1831, p. 379), by original designation. Recent, in estuaries and brackish lagoons, Indo-Pacific. Systematic palaeontology Diagnosis (emended from Symonds and Pacaud, 2010, p. Superorder Neritimorpha Golikov and Starobogatov (1975, 59) – Clithon with small smooth shell; septum weekly p. 200) arched with one large and several small teeth. Order Cycloneritimorpha Bandel and Frýda (1999, p. 220) Family Neritidae Rafinesque (1815, p. 144, as Neritinia) Discussion – Although Holthuis (1995, pp. 196-197) syn- Genus Clithon de Montfort (1810, p. 327) onymised Pictoneritina under the nomenclaturally senior genus Clithon on anatomical grounds, Symonds (2006, p. Type species – Nerita corona Linnaeus (1758, p. 777, nr. 31) following Wenz (1938, p. 423) retained Pictoneritina 629), by original designation. Recent, fresh to brackish as a subgenus of Clithon in view of the substantial mor- water, Eastern Indian Ocean to the Southwestern Pacific. phological differences between the shells of the two type species. Clithon (Clithon) corona is considerably larger Diagnosis (emended from Symonds and Pacaud, 2010, than C. (P.) oualaniense with a thick periostracum and p. 59) – Neritid with small spire and large body whorl, usually a row of prominent spines around the shoulder some species with a subsutural row of spines; labial area whereas C. (P.) oualaniense is smooth and glossy with- smooth with one or more teeth on the margin; opercu- out spines or periostracum and with elaborate colour pat- lum smooth or bearing minute granules on the exterior terns. There are in excess of fifty generally recognised 148 Symonds. New species of Neritidae from the Solent Group (late Eocene and early Oligocene) of the Hampshire Basin species of extant Clithon (Eichhorst, in prep.) of which two and a quarter whorls, semiglobular with an exserted only about ten are Pictoneritina. All extant species of spire. The protoconch is smooth, obovate and approxi- Clithon (Pictoneritina) are small, smooth, glossy, with- mately 0.3 mm wide. The exterior of the teleoconch is out spines, lacking a discernable periostracum or with glossy and covered by numerous fine, closely spaced col- only a very thin transparent one, with distinctive colour labral ridges. The suture is impressed and the whorls are patterns and with one large and several small teeth on convex and rather shouldered. the septum edge. Extant species of Clithon (Clithon) are The colour pattern is very variable consisting of dark generally larger, usually with a more or less rough tex- brown lines on a pale background. Some specimens are tured shell, often with spines, the colour pattern wholly almost entirely white with only a narrow band of mark- or partially obscured by a periostracum and the teeth on ings above and below the suture (paratype 3); in others the septum edge similar to Clithon (Pictoneritina) but the brown lines coalesce leaving between them a series more variable. There is no single characteristic which of pale dots of irregular size and shape (paratype 2) and can be used to differentiate the two subgenera but when in a few examples the dots are hardly noticeable and the all these morphological features are taken into account whole teleoconch appears brown (paratype 5). Clithon (Pictoneritina) can be readily separated from The aperture is oblique, semicircular and broad. The Clithon (Clithon). septum is flat and almost smooth; the edge concave. The The new taxa herein described have been compared with dentition on the septum edge is indistinct, many speci- the other species of Cenozoic Pictoneritina from north- mens appearing edentate, where present it consists of west Europe. one broad, shallow tooth, at about a third of the distance from the apical end, with up to six small, irregular and scarcely defined teeth abapical to it. The outer lip is thin Clithon (Pictoneritina) pococki nov. sp. and smooth within. The apertural tooth is in the form of Pl. 1, figs 1-6 a narrow oblique ridge below the abapical end of the sep- tum which does not project beyond the septum edge. 2006 Clithon (Pictoneritina) concavus (J. de C. Sowerby, The outer surface of the operculum is slightly concave 1823) – Symonds, p. 33, figs 9-12 [notClithon (Pic­ and smooth apart from faint growth lines. On the inner toneritina) concavum (J. de C. Sowerby, 1823)]. surface a shallow groove runs in an arc from the nucleus to the middle of the columellar edge. The ventral apophy- Type material – Holotype NHMUK PI TG 26755 (Pl. 1, sis consists of a prominent curved ridge projecting be- figs 1a-c), height 6.6 mm, width 6.4 mm. Paratype 1 oper- yond the edge of the operculum with the well-developed culum (broken) NHMUK PI TG 26756 (Pl. 1, figs 2a-b), dorsal apophysis at right angles to it and joined to it for width 2.8 mm. Paratype 2 NHMUK PI TG 26757 (Pl. approximately half its length by a thin calcareous callus. 1, figs 3a-c), height 7.0 mm, width 7.0 mm. Paratype 3 There is a pit at the base of the dorsal apophysis. NHMUK PI TG 26758 (Pl. 1, figs 4a-c), height 6.0 mm, width 5.8 mm. Paratype 4 NHMUK PI TG 26759 (Pl. Distribution – Apart from the type horizon, Clithon (Pic­ 1, figs 5a-c), height 4.8 mm, width 5.2 mm. Paratype 5 toneritina) pococki also occurs slightly higher in the Tot- NHMUK PI TG 26760 (Pl. 1, figs 6a-c), height 5.2 mm, land Bay Member in the ‘Cyrena pulchra Bed’ at Headon width 5.2 mm. All from the type locality, (all Malcolm Hill: paratype 5; and lower in the member in greenish Symonds leg.). Paratypes 1-4 from type stratum, para- marl underlying the ‘Brockenhurst Bed’ at Whitecliff type 5 from Totland Bay Member of the Headon Hill For- Bay, Isle of Wight, 50°40’18”N 1°05’41”W (estimated mation, ‘Cyrena pulchra Bed’. from Google Earth) (Bed F of Stinton, 1971, p. 405): NHMUK PI TG 26761 (Malcolm Symonds leg.) and in Type locality – Headon Hill, Isle of Wight, England, the Colwell Bay Member of the Headon Hill Formation 50°40’16”N 1°34’09”W (estimated from Google Earth). in the ‘Milford Marine Bed’ at Milford, Hampshire, Eng- land, 50°43’35”N 1°36’51”W (estimated from Google Type stratum – Late Eocene (Priabonian), Headon Hill Earth): NHMUK PI TG 12267-9 (D. Curry leg.). Formation, Totland Bay Member, green clay between two limestone bands, the upper of which is immediately be- Other material studied – Approximately six hundred and low the ‘Cyrena pulchra Bed’. sixty specimens were examined from the type horizon, a further six hundred from the ‘Cyrena pulchra Bed’, Etymology – Named after my colleague William Pocock, eighty-five from the marl below the ‘Brockenhurst Bed’ a molluscan palaeontologist who has accompanied me on at Whitecliff Bay and twenty from the ‘Milford Marine many field trips to the Isle of Wight. Bed’ (all Malcolm Symonds private coll.) Diagnosis – A medium size Pictoneritina with an exsert- Discussion – In the type stratum there is considerable ed spire, two and a quarter teleoconch whorls, which are lateral variation in distribution with Clithon (Pictone­ somewhat shouldered, an almost edentate septum edge ritina) pococki common in some parts of the bed and and a very variable colour pattern. absent in others (pers. obs.). Opercula were obtained by soaking a large number of shells of C. (P.) pococki in hot Description – A medium-sized shell consisting of about water to loosen the matrix, agitating them to clean out Cainozoic Research, 15(1-2), pp. 147-153, October 2015 149 the apertures and passing the resulting residue through er than paratype 1. The protoconch is covered by the first a fine sieve; a method suggested by Alan Morton (pers.