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 



