F. Hardy — Mineral Composition of Fenland Silt. 543 



in the ibex-zone), together with Lipnroceras, and which attained 

 considerable size, in the genus Polyniorphites. It appears to be 

 a late and independent development of the Polyniorphidse, and 

 possibly local, since nothing like the new forms has been recorded 

 from such well-searched localities as e.g. Wurtemberg. It may be 

 added that though the ellipticum group of Tropidoceras, with all 

 its variations, and possibly some Tragophylloceras of the Wechsleri 

 group, confused with Amqjhiceras, occur in Sicily, and both JJptonia 

 jamesoni and Acanthopleuroceras valdani are widely distributed 

 in the Mediterranean province also, there apparently is nothing com- 

 parable to the new genus Dayiceras, or, indeed, to the typical post- 

 jamesoni genus Polymorphites. 



EXPLANATION OF PLATE XV. 



Figs. 1-4. — Dayiceras polymorphoides nov. gen. et .sp. Lias (Pliensbachian, 

 ibex-zone), Charmouth. Specimen No. Z 216, Britisli Museum, Natural History- 



Fig. la. — Side-view of holotype, nearly natural size. 



Fig. 16. — Restored outline section. 



Fig. Ic. — Portion of suture-line, magnified. 



FtG. 2. — Portion of periphery, magnified, to show limonitic outer shell (dark), 

 inner shell or septum, also limonitic (gray), and pja'itic cast (two 

 ribs on left, clear). 



Fig. 3. — Peripherj^ restored from evidence of several specimens. 



Fig. 4.— Rib-curve, with strong forward bend at umbilical slope. 



Fig. 5. — Dai/icera.s lan-ji nov. Side-view, nearly natural size. Lias 

 (Pliensbachian, ibex-zone), Charmouth. Specimen No. C 18835, Dan Haggard 

 Coll , British Museum, Natural History. No sectional view is given, since the 

 specimen is crushed. 



The Mineral Composition of the Modern Fenland Silt, 

 with special reference to Carbonate Minerals. 



By F. Haedy, M.A. (School of Agriculture, Cambridge) 



I. INTEODUCTORY 



TN July of 1919 the writer assisted in a soil survey of the salt 

 -*- marshes of the north-west shores of the Wash. 



During the course of the work it became necessary to examine 

 microscopically specimens of marine silts forming the salt marshes, 

 with a view to elucidating the reason why the silts generally have 

 an abnormally high carbonate content. 



It was found that the carbonate nrincrals in the material examined 

 were of several kinds Amongst them dolomite (MgCOg. CaCO.,) 

 was definitely identified. This discovery suggested a line of work 

 both of geological and mineralogical interest, and also of possible 

 agricultural value, for dolomite is a carbonate which is not affected 

 appreciably by weak acids (such as those that are said to be liberated 

 in soils by the decay of humus), and it seemed reasonable to suppose 



