32 THE 'derived' fossils. 



of our present sea Hues. There they were broken into fragments 

 and rolled into pebbles till they found a resting-place and were 

 buried in the accumulating sediment together with the true abori- 

 ginal shells that had lived in the Neocomian sea. Lithologically 

 I am not able to distinguish the derived Ammonites Deshayesii of 

 the Neocomian from the Am. biplex of the upper Jurassic. 



The following is a list of the phosphatised Neocomian derived 

 species: Ammonites Deshayesii, Leym.; Ammonites, smooth species; 

 Ancyloceras, sp.; Hamites,^.; Gervillia linguloides, Forbes? Thetis 

 minor, Sby.; Terebratula ovoides, Sby.; Terebratula ovoides, var. 

 rex, Lankester. 



At Potton we meet with the same list of species with, in addi- 

 tion, fragmentary phosphatised specimens of Ancyloceras gigas. 

 In the Tock of Heligoland similar, though usually smaller, frag- 

 ments of the same Ancyloceras are found upon the shore. But the 

 richest of these derived phosphatised Neocomian faunas is that of 

 Hunstanton, where the following species were collected for the 

 Woodvvardian Museum: Perna Mulleti (some of them derived, but 

 some look like natives); Pleurotomaria; Ammonites Cornuelianus 

 Forbes ; Ammonites M artini '(Forbes) ; Ammonites Deshayesii (Leym.) ; 

 Ammonites, sp. ? (allied to Koenigi); Ammonites, sp. 2; Ancyloceras 

 gigas (Sow.); Ancyloceras (tuberculated species); Nautilus, sp. 



B. The Dark-grit Types of Derived Neocomian Fossils. 



A peculiar, dark-coloured grit rock, containing a special fauna, 

 occurs at Upware in the form of derived pebbles and boulders. 

 The same rock is found in precisely similar condition in the Potton 

 sands of Bedfordshire and the lower red sands of Hunstanton, in 

 Norfolk. 



The Upware and Potton blocks may be described as hard, 

 ragged-looking lumps of most irregular form but with eroded 

 surfaces: the unevenness is produced by the fossils, which here 

 project into angles, and there leave hollows where they have fallen 

 away. It is a moderately fine-grained stone, the constituent 

 particles being quartz and ironstone, cemented together with iron 

 oxide. It may be to some slight extent impregnated with phos- 

 phate. Generally it is crowded with casts of fossils. The Upware 

 blocks were small, and yielded the following species: Cerithiwm 



