42 THE 'DERIVED' FOSSILS. 



Swindon Quarry will readily recognise the same rock and fossils 

 (though now somewhat changed by being impregnated with phos- 

 phate of lime) among the Brickhill pebbles. Its specially 

 characteristic fossil is the Astarte cuneata of Sowerby, arid with 

 these, associated in the same blocks, occur Litorina sp., Trigonia 

 (small), Lucina sp., Myacites (very inequilateral sp.), Natica, Litho- 

 domus, and Ammonites biplex ; the latter with the pearly lustre of 

 its shell well preserved. 



Several of the Portlandian species mentioned below (e.g. 

 Cardium dissimile, Trigonia gibbosa and Lucina Portlandica) 

 may also have been derived from the same bed, for they are all 

 found together in the Limestone of the Swindon series. 



The rock as now found at Brickhill is a rather pale chocolate- 

 coloured phosphate, with many coarse grains of sand. 



To the freshwater beds of this same series I would refer the 

 many fragments of silicified wood found at Potton and Brickhill. 



From Upware the evidence of this series is less satisfactory. 

 Some much rolled and worn casts of bivalves may pertain to the 

 Astarte cuneata, but this is very doubtful, and I cannot consider 

 that we have any conclusive evidence of the former existence of 

 any part of the infra-cretaceous fluviomarine series, either Port- 

 landian, Purbeck, or Wealden in this area. 



B. The Portlandian Fossils. 



The Portlandian species that have been recognised (some of 

 them doubtfully) at Brickhill are the following : Ammonites 

 giganteus (Sby.) ; Ammonites biplex (Sby.) ; Myacites; Modiola, 

 sp. ; Cyprina; Cardium dissimile (Sby.); Myoconcha Portlandica 

 (Blake) ; Trigonia gibbosa (Sby.) ; Trigonia incurva (Sby.) ; Pho- 

 ladomya tumida (Ag.) 



Buccinum naticoides and Lucina Portlandica occur, I believe, 

 at Upware. 



Mr Walker has pointed out to me that many of these fossils 

 (Cardium dissimile, &c.) occur in a very gritty phosphate, but 

 they cannot be easily separated off from the other derived fossils 

 by lithological characters. 



I am inclined to think that a large proportion of the derived 

 species of the Upware and Brickhill Neocomians were washed out 

 of old coast lines made up of these uppermost Jurassic rocks. 



