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 A HISTORY OF NORTHAMPTONSHIRE 1870 and 1873. Full lists of all the fossils then known from each formation are there given ; and those desirous of taking up the subject in earnest should refer to these papers. For the benefit of those who have but little acquaintance with geology it may be mentioned that the abundance of ammonites and belemnites in the Northamptonshire rocks at once indicates that they belong to the Secondary or Mesozoic period, while the prevalence of the bivalved mollusc Trigonia, together with other characteristic forms, indicate that they pertain to the Oolitic or Jurassic, rather than to the Cretaceous system. Among the characteristic fossils of the Oxford Clay may be noted the great hemispherical oyster known as Gryphaa dilatata, which in this county seems to be confined to the lower and middle beds of the forma- tion. In the same beds occurs Belemnites oiveni, which is the largest species of its kind ; while higher up in the series this is replaced by the smaller B. hastatus. The topmost clays abound in Ammonites cor datus and A. ornatus ; each of which is, however, confined to a separate zone. Their shells, when first exposed to the air, have a brilliant golden lustre, from the presence of pyrites ; but the action of the oxygen on this mineral causes them quickly to perish and disintegrate. A mussel- like shell {Avicula incequivalvis), in which one valve is smaller than the other, is very characteristic of the lowest beds of the Oxford Clay, which are more or less sandy. In the Cornbrash at Rushden, south of Higham Ferrers, the fossil collector will find a ' happy hunting-ground.' The place of Gryphaa dilatata is taken by the large furbellated Ostrea marshi, which is a true oyster of easy recognition. More characteristic is, however, the large bivalve Pholadomya bucardimn other bivalves found in this formation being Avicula echinata, Gresslya peregrina, and Myacites securiformis and decurtatus. Brachiopods are represented by Terebratula obovata^ T. lagen- alis, and Rhynchonella concinna ; while the small sea-urchin known as Echinobrissus clunicularis is a common fossil of the Cornbrash. Fossils are rare in the clays of the Great Oolite, the most charac- teristic being the oyster-like Placunopsis socialis. On the other hand, the limestones of the same series are highly fossiliferous, and, as is generally the case, contain a large proportion of gastropod molluscs, although these are less numerous than at Minchinhampton. The gastropods include representatives of the extinct genera Alaria, Amberleya, Nerincea, Chemnitzia, and Trochotoma, together with species of the still existing Natica, Phasianella, and Pleurotomaria. Cephalopods are less common, but include Ammonites gracilis and macrocephalus. Nautilus babert, and a belemnite. Among the more characteristic bivalves may be mentioned a scallop, Pecten lens, the cockle Cardium stricklandi, the Area - like Cuculcea concinna and Macrodon kirsonensis, several species of the common Oolitic genus Pholadomya^ such as Ph. acuticostata and deltoidea, together with Trigonia costata and moretoni. With the exception of Macrodon, which is very close to Cuculcea, all the above-mentioned genera of bivalves have living representatives ; but the Great Oolite of the 44