. Bulletin of the British Museum (Natural History), Geology. OF NORTH-WEST EUROPE 303 Diagnosis: A member of the superspecies C. laticosta with an evenly rounded anterior margin, convolute dorsal ridge and small areas of coarse puncta around the median and ventral ridges. Material: Three carapaces from the Lower Barton Beds of Alum Bay; numerous specimens from the Middle and Upper Barton Beds of Barton, and Alum Bay; fifteen valves and carapaces from the Mames a P. ludensis of Verzy, and five from Chaven^on. lo 3864-70. Type locality and horizon: Barton Clay of Barton (see discussion). Stratig
. Bulletin of the British Museum (Natural History), Geology. OF NORTH-WEST EUROPE 303 Diagnosis: A member of the superspecies C. laticosta with an evenly rounded anterior margin, convolute dorsal ridge and small areas of coarse puncta around the median and ventral ridges. Material: Three carapaces from the Lower Barton Beds of Alum Bay; numerous specimens from the Middle and Upper Barton Beds of Barton, and Alum Bay; fifteen valves and carapaces from the Mames a P. ludensis of Verzy, and five from Chaven^on. lo 3864-70. Type locality and horizon: Barton Clay of Barton (see discussion). Stratigraphical range and distribution : Barton Clay of Barton, Alum Bay and Whitecliff Bay; Marnes a P. ludensis of Verzy and Chaven9on; Argile d'Asche of Oedelem (Belgium). Description : Sex ratio, I : i-6. The carapace is massive, the shell is thick. The dorsal ridge is thick with convolutions forming some six depressions, which bear normal pore canals. The median ridge has a prominent swelling in the central part and has an area of coarse puncta around the four depressions. The ventral ridge likewise has an area of coarse puncta. The inner margin has a narrow and deep anterior indentation, particularly in the right valve, and a prominent ventral one. The anterior segment is sharply rounded. The median and posterior segments are inseparable; they slope up to the posterior indentation, which is quite close to the ventral margin. Dimensions: Left valve Right valve L H L/H W L H L/H Female o-8o ± 0-02 0-50 ± o-oi i-6o ± 0-04 0-46 079 0-42 i-88 Male 0-86 ± 0-02 0-49 ± o-oi 176 ± 0-04 0-45 0-89 0-46 1-94 Discussion: Reuss described this as coming from the London Clay of Barton, Hampshire. This relates to the idea prevalent in the early 19th century that the clay at Barton was of the same age as that at London; in fact, the London Clay is Lower Eocene, while the Barton Clay is Upper Fig. 15. Cytheretta laticosta (Reuss) showing distribution of normal pore canals in relatio
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