on the Organic Composition of' Chalk and Chalk Marl. 311 



The thirty-nine species of siliceous-shelled Infusoria belong 

 to fourteen genera, and are as follow: — 



Ac\\nocyc\ns ternarius, A.-''qn(ite?narius, A. ^quitiarius, A. 

 sctiarius, A. septenarius, A. octonarius, A. denarius', Coccone- 

 ma Cretcc', Cornutella clathrata\ Coscinodiscus Argus^ C. 

 centralis, C lineatus, C ''^minor, C* Patina', Denticella i^ra;- 

 gilaria, D. tridens', Dictyocha Fibula, D. Navicida, T). jwlj/- 

 actis, D. speculum, D. stella, D. triangtda ; Euiiotia zebra ; 

 Fragilaria rhabdosoma, F. striolata?; GaWioneUn aurichalca?, 

 G. sulcata; Haliomma Medusa, H. crenalum; Lithocampe 

 lineata, L. Radicnla, L. solitaria ; Navicula ajricana, N. Ba- 

 cillum, N. eurysoma, N. ventricosa, N. sicida ; Pyxidicula 

 prise a ; Synedra idna. 



The seven species of soft-shelled Infusoria of the flints be- 

 long to three genera, and are the following: — Chaetophyta 

 Pyrites', Peridinium /?yrop/zo?7^w?f ; Xanthidium bulbosum, X. 

 furcatum, X. hirsutum, X. ramosum, X. tubiferum. 



The five species of siliceous plants belong to two genera, 

 namely, Spongia {Tethya'^) aciculosa, S. cancellata, S. '^Cri- 

 hrum, S. binodis', Spongilla {Tethyal) lac2istris\. 



Of these principal forms the before-mentioned rocks partake 

 in the proportions as stated below : namely. 



+ Peridinium delitiense has hitherto been found only in flint pebbles 

 near Delitzsch, yet accompanied with forms that are common in the flints 

 of the chalk. 



+ In the preceding lists, the species which ate marked with an asterisk * 

 are those which most frequently occur, forming the masses of the rocks. 

 The Rotalia globulosa occurs in ail the localities. 



