PHOSPHATES IN THE CAMBRIAN ROCKS. 371 



proportion of phosphate of lime in each is exceedingly small. Re- 

 cent shells of Mollusca also, as a rule, only contain it in very small 

 quantity ; while the recent Crustacea, such as the lobster, crab, and 

 sea-crayfish, contain in their shell on an average from 6 to 7 per cent, 

 of phosphate of lime. The animal matter of the latter is also known 

 to contain a considerable amount of phosphoric acid. Mr. Hudle- 

 ston's analysis of the flesh of the recent lobster gives 0*332, and of 

 the lobster-shell 3-05 per cent of P 2 5 . Doubtless also the animal 

 matter of the Mollusca contains phosphoric acid in. some quantity ; 

 for the matrix in which some of the fossil shells are imbedded, and 

 the casts I find, frequently contain more than is to be found in the 

 shell itself. The great Crustacean (Paradoxides) which occurs in 

 such abundance in the Menevian rocks, contains in its fossil shell 

 as much as from 17 to 20 per cent, of P 2 0., and hence a far greater 

 amount than is found generally distributed throughout the rock in 

 which it occurs. Doubtless this large Crustacean, in some cases 

 over 20 inehes in length, was also fully capable of producing all 

 that is now present in these rocks ; for we have no evidence of the 

 existence of any other forms of animal life at this period which 

 could produce it in any very considerable proportion. The Mollusca 

 were very small and scantily distributed. The remains of sponges 

 occur only as fine siliceous or pyritized spicules ; and there is no 

 evidence of much vegetable matter. We are therefore compelled to 

 fall back on the Crustacea, which occurred in myriads in these seas, 

 and which, having the power of casting off their shells frequently, 

 must have added greatly to the deposits. Indeed most of the car- 

 bonate of lime, which is here also very abundant (being, according 

 to Mr. Hughes's analyses, in the proportion of as much as 42-39 

 per cent, in some portions of the beds containing P. Davidis), was 

 also in all probability deposited by them. In many of the little 

 phosphatic nodules which are interspersed throughout these deposits, 

 casts of bivalve Crustacea are frequently seen enclosed, thus making 

 it probable that these nodules were produced chiefly through the 

 decomposition of the soft tissues and shells of these animals. 



It is evident that the shell of the Trilobite was thicker and 

 contained a larger proportion of phosphate of lime than those of the 

 other Crustacea which lived at this time ; for in the rocks where the 

 phyllopod Crustacea, for instance, alone occur (as the Lingula-flags 

 with Hymenoearis and the Skiddaw slates with Caryocaris*), the 

 proportion of phosphate and also of carbonate of lime is much less 

 than where Trilobites occur alone. I can only account for this by 

 supposing that the shell of these phyllopod Crustacea must have 

 been thinner and less rich in calcareous material ; for where they 

 occur they are as plentiful as are the Trilobites in some of 

 their richest beds. 



I have marked in the section (p. 372) the position of the beds 

 in the " Menevian group " at Porth-y-rhaw which have already 

 been examined for phosphoric acid. The degree in which it occurs 



* Which I have been able to examine, through the kindness of Mr. Etheridge, 

 F.R.S. 



