224 PROCEEDOGS OF THE GEOLOGICAL SOCIETY. [Jan. 22, 



been so amply shown by Professor Ramsay*, a great unconformity 

 between two series of beds is always accompanied by a striking dis- 

 crepancy between their fossil contents. Such a discrepancy, how- 

 ever, is not found between the faunas of the Upper Greensand and 

 Gault, but is well known to exist betweeu those of the last-men- 

 tioned formation and the Neocomian. 



Now let us turn to the direct palgeontological evidence for refer- 

 ring the Speeton Clay to the age of the Gault. Professor Phillips 

 gives the following list of fossilsf as common to the two formations : — 



Ammonites planus ? Mant. 

 Hamites intermedins ? Sow. 



rotundns, Sow. 



attenuatus, Mant. 



• alternatus, Mant. 



plicatilis, Mant. 



Belemnites minimus, List. 



Eostellaria composita, Sow. 

 Nucula ovata, Mant. 

 Pholadomya decussata, Mant. 

 Yermievdaria Sowerbyi, Mant. 

 Spatangus argillaceus, Phill. 

 Caryophyllia cornulus, Fhill. 



The specimens referred to Am. )jlanus, Mant. (which is only one 

 of the numerous varieties of A. splendent), have now been long 

 recognized as certainly not referable to the Gault species, but as 

 belonging to a well-known Neocomian form, the Ammonites Nisus of 

 D'Orbigny. ( Vide Morris's Catalogue and D'Orbigny's Prodrome.) 



The various species figured by Young and Bird, Phillips and Eomer, 

 as Hamites (generally from very small fragments only) I shall show 

 in the sequel to be really referable to the genus Ancyloceras, and 

 usually to well-known IS'eocomian species and varieties of that genus. 

 ( Vide Appendix B.) 



Belemnites minimus, I believe, really occurs in the Speeton Clay, 

 but rarely, while in the Gault and Hunstanton limestone its pro- 

 digious abundance is most striking. 



Rostellaria composita is probably a misprint for B.. Parhinsoni, 

 under which name, as Professor Forbes has shown, a number of 

 species, both Gault and l^eocomian, have been confoundedj. 



Nucula ovata. — Professor Phillips's type specimens of this species 

 are both young shells. I have had the opportunity of examining a 

 number of specimens of all ages, and believe that they are referable 

 to N. planata, Desh., which is probably only a variety of N. ohtiLsa, 

 Sow. 



Pholadomya decussata. — The type specimen is so crushed and 

 distorted that it is impossible to form any certain judgment as to 

 its affinities. There is but little resemblance between it and Man- 

 tell's species from the Chalk-marl ; and I believe it to be the Phola- 

 domya Martini, Yovhes, a shell which is certainly not uncommon in the 

 Speeton Clay. « 



VermiculaHa Sowerbyi . — It is difficult to form a judgment on the 

 agreement of th« species so abundant in the Speeton Clay with that 

 figured by Mantell from the Chalk-marl, on account of the im- 

 perfect character of the drawing in 'The Geology of Sussex.' 



* Anniversarv Addresses to the Greological Society, 1863, 1864. 

 t Geology of Yorkshire, 2nd ed. (1835) p. 350. 



* Quart. Journ. Geol. Soc. vol. i. (1845) p. 350. 



