68 MR. H. WOODS ON THE MOLLUSCA [Feb. 1896^ 



6. The Mollusca of the Chalk Eocz : Part I. By Henry Woods, 

 M.A., F.G.S. (Eead December 4th, 1895.) 



[Plates II.-IV.] 



Contents. 



Page 

 I. Introduction and Literature 68 



II. Cephalopoda 73 



Nautilus. Prionocyclus. 



Ptychoceras. Pachydiscus. 



Hamites. Scaphites. 



Heteroceras. Crioceras. 



Baculites. 



III. Gasteropoda ■ .•■ 85 



Dmarginula. Natica. 



Pleurotomaria. Cerithium. 



Trochus. Aporrhais. 



Turbo. Lampusia ? 



Crepidula. Avettana. 



IV. Scaphopoda 96 



V. Tables of Distribution 94,95 



I. Introduction and Literature. 



The Chalk Eock is a bed of hard, usually cream-coloured limestone,, 

 occurring at the top of the Middle Chalk ; it succeeds the zone of 

 Holaster planus and is followed by the zone of Micraster cor-testudi- 

 narium. 1 Noticeable features in the bed are the presence of grains 

 of glauconite and numerous green-coated nodules of phosphatic 

 chalk. Under the microscope it is characterized by the abundance 

 of foraminifera, especially Globigerina, and by numerous fragments 

 of the tests of mollusks and echinoids ; the bright green grains of 

 glauconite are also conspicuous. The insoluble residue, obtained by 

 treating with hydrochloric acid, was found by Dr. W. F. Hume to 

 consist mainly of quartz and glauconite ; augite, hornblende, and' 

 tourmaline occur very rarely. Frequently the Chalk Eock is not 

 hard throughout, but contains irregular cavities filled with soft 

 mealy chalk or with red sandy clay. Analyses of the rock have 

 been made by M. Duvillier, 2 Dr. Frankland, 3 Mr. C. W. Meanwell 

 and Mr. P. G. Sandford, 4 and they show from *20 to '713 per cent. 



1 In this paper I have adopted the zones given by Mr. Jukes-Browne in 

 Whitaker's 'Geology of London' etc., vol. i. (1889) p. 58 (Mem. Geol. 

 Survey). 



2 Barrois, ' Description geologique de la Craie de l'lle de Wight,' Ann. 

 Sciences Geol. vol. vi. (1875) p. 19. 



3 Penning and Jukes-Browne, ' Geol. of the Neighbourhood of Cambridge, 

 Mem. Geol. Surv. (1881) p. 66. 



4 « Handbook of the London Geol. Field Class ' (1892), pp. 120, 121. 



