Fig. 156: Leda (Nucula) Deshayesiana. Lower Miocene, Belgium.—The Upper Miocene Bolderberg beds, mentioned in p. 224, rest on a Lower Miocene formation called the Rupelian of Dumont. This formation is best seen at the villages of Rupelmonde and Boom, ten miles south of Antwerp, on the banks of the Scheldt and near the junction with it of a small stream called the Rupel. A stiff clay abounding in fossils is extensively worked at the above localities for making tiles. It attains a thickness of about 100 feet, and though very different in age, much resembles in mineral character the “London clay,” containing, like it, septaria or concretions of argillaceous limestone traversed by cracks in the interior, which are filled with calc-spar. The shells, referable to about forty species, have been described by MM. Nyst and De Koninck. Among them _Leda_ (or Nucula) _Deshayesiana_ (see Fig. 156) is by far the most abundant; a fossil unknown as yet in the English tertiary strata, but when young much resembling Leda amygdaloides of the London Clay proper (see Fig. 213). Among other characteristic shells are _Pecten Hœninghausii,_ and a species of _ Cassidaria,_ and several of the genus _Pleurotoma._ Not a few of these testacea agree with English Eocene species, such as _Actæon simulatus,_ Sowb, _Cancellaria evulsa,_ Brander, _Corbula pisum_ (Fig. 157), and _Nautilus (Aturia) ziczac._ They are accompanied by many teeth of sharks, as _Lamna contortidens,_ Ag., _ Oxyrhinaxiphodon,_ Ag., _Carcharodon angustidens_ (see Fig. 196), Ag., and other fish, some of them common to the Middle Eocene strata. _Kleyn Spawen beds._—The succession of the Lower Miocene strata of Belgium can be best studied in the environs of Kleyn Spawen, a village situated about seven miles west of Maestricht, in the old province of Limburg in Belgium. In that region, about 200 species of testacea, marine and fresh-water, have been obtained, with many foraminifera and remains of fish. In none of the Belgian Lower Miocene strata could I find any nummulites; and M. d’Archiac had previously observed that these foraminifera characterise his “Lower Tertiary Series,” as contrasted with the Middle, and they therefore serve as a good test of age between Eocene and Miocene, at least in Belgium and the North of France.[3] Between the Bolderberg beds and the Rupelian clay there is a great gap in Belgium, which seems, according to M. Beyrich, to be filled up in the North of Germany by what he calls the Sternberg beds, and which, had Dumont found them in Belgium, he might probably have termed Upper Rupelian.