Nodules of the Trimmingham Chalk. 441 



After having explained a difficulty with regard to the 

 intrinsic view, one may en revanche suggest one to the 

 extrinsic view. This is to be found in the restriction of the 

 flints to definite layers in the chalk, the chalk above and below 

 being free both from them and from sponge-spicules. It is 

 difficult to see, in the first place, how a shallow sea came to 

 consist of a strong solution of silica, and still more so to under- 

 stand how it came to vary in a rhythmical fashion, sometimes 

 being concentrated enough to lead to the formation of flints, 

 and again pure enough to leave the intervening chalk almost 

 absolutely devoid of silica. 



(ii) The accumulation of the Sponge-spicules. — Since we 

 have shown that the silica of the flints has in all probability 

 been derived in many cases from accumulations of sponge- 

 spicules, we have next to show how these accumulations were 

 produced. Two different explanations naturally suggest 

 themselves : either the spicules have been derived from suc- 

 cessive generations of sponges which grew upon the same 

 spot, or they have been separated from a large quantity of 

 chalk and washed together by current-action. 



The Trimmingham flints contain each a diversified collec- 

 tion of spicules derived from several different genera of several 

 different families of sponges ; and the assemblage of forms 

 obtained from one flint does not differ in any distinct way 

 from the assemblage obtained from another. This possession 

 by a number of separate flints of a group of diverse spicules 

 in common might lead us at first to suppose that the spicules 

 had been drifted together by currents, except that such a 

 supposition would not account for the characteristic form 

 presented by many of the flints. Of this curious association 

 of well-preserved external form with a mixture of spicules the 

 chalk affords numerous striking examples, of which, perhaps, 

 the best-known is that of Cceloptychium* ', which yielded to 

 Zittel quite as many extraneous spicules as are figured here 

 from the Trimmingham nodules, and which, at the same time, 

 presents us with a much more characteristic external form. 

 Another, less known, is that of the so-called Neptune's cup 

 of the chalk : this is incontestably the Cretaceous representa- 

 tive of the existing Poterion patera, Hardwicke ; so that it 

 may well be named Poterion cretaceum. It is a suberite sponge 

 with characteristic outer form, and when alive contained only 

 pin-headed spicules ; in its silicified state, however, it is 

 crowded with other forms, which have been introduced into 

 it from without. Poterion cretaceum has certainly not been 

 drifted, it has been silicified where it stands ; and so, we 

 * Abkaudl. der k. kayer. Akad. der W., h. CI. xii. Bd. iii. 1876. 

 Ann. & Mag. N. Hist. Ser. 5. Vol. vi. 31 



