in Flints of the Chalk. 193 



infusoria interspersed with the larger univalve. That the ap- 

 parently inorganised particles are derived from the decompo- 

 sition of the Cypris will scarcely be doubted, and to what ex- 

 tent each individual is capable of yielding a supply of calca- 

 reous matter is easily ascertained by incinerating recent ex- 

 amples. For it thus appears, that not only is there an inde- 

 structible though slender shell covering the body properly so 

 called, but the delicate branches of the rami or arms inserted 

 on each side of the head, as well as the arms themselves, are 

 equally supplied with a frame-work of solid matter. 



But my present object is to allude more particularly to 

 some of the fossil contents of flint pebbles and of the flint no- 

 dules of chalk. It is now well known that flint of every kind 

 is rich in organic remains, and few persons who use the mi- 

 croscope at all, have neglected the examination of these minute 

 forms which had their little moment of life and enjoyment in 

 ages of the most remote antiquity. Perhaps, however, it is 

 not so generally understood, that in the hands of a skilful 

 geologist a promiscuous series of flint pebbles would be as- 

 signed, with the utmost precision, and by means of their fossil 

 contents alone, to their proper periods and strata. Yet such 

 is the fact, and I have had the pleasure of seeing it verified by 

 my friend Mr. Bowerbank, who lately took advantage of a 

 geological tour to establish this curious result. I had, indeed, 

 myself suspected that the flint of different strata had not a 

 common origin, in consequence of the absence of the Xanthi- 

 dium from many of the pebbles of the Brighton beach*. This 

 highly interesting animalcule, of which several species occur 

 in the flint of Kent and Surrey, I discovered first of all about 

 a year ago in the flint of the Sydenham gravel ; and this spe- 

 cimen was compared and identified with a French one, then 

 but just imported at an expense exceeding 20 francs. I learnt 

 on that occasion that Prof. Ehrenberg had already named and 

 described this new fossil genus, and to him I am indebted for 

 the names of the species which accompany this paper. Of the 

 beauty of the drawings it is unnecessary to speak, and their 



* One of these pebbles abounds with remarkably fine examples of Pyxi- 

 dicula, and its crystalline state, by no means common to flint nodules, is de- 

 cidedly proved by its action on polarized light. 



Ann. Nat. Hist. Vol. 2. No. 9. Nov. 1838. o 



