THE SHELL-LIMESTONE OF DURHAM. 127 



sons for viewing it as an accumulation in comparatively deep 

 water. Its litliological characters, and the general features of 

 its fauna, favour such an opinion. And though the generic rela- 

 tions of the Eiitomostraca do not offer strong support in either 

 instance, yet, in the case of the shell-limestone, the prevalence of 

 Bairdice harmonises with the supposition of their habitat having 

 been in deep water, for I am informed by Mr. Jones that recent 

 BairdlcB are often found at great depths. These remarks are made 

 rather as suggestions than positive opinions; for the subject of 

 physical conditions, at the period of deposition of these lime- 

 stones, is too important and difficult of solution to be treated of 

 incidentally. For the present, I merely suggest that the slight 

 differences observed in the generic features of the two groups of 

 Entomostraca, may be due to a difference of depth of the ocean 

 in which they lived. 



For the convenience of those who may find fossil Eiitomos- 

 traca, or other small organisms, in a similar matrix to that in 

 which the Tunstall specimens occurred, it may be well to add a 

 word or two as to the mode I adopted for extracting the Per- 

 mian specimens from the calcareous dust. In the first place, I 

 sifted the dust of all the coarser particles — of everything larger 

 than the tenth of an inch; and, from what was left, I took all 

 the very fine dust with another sieve, leaving a residue, among 

 which everything organic could easily be distinguished.* In 

 picking out the organisms, a piece of polished slate — a common 

 school-slate, for instance — is a good area on which to strew a 

 portion of the residue for examination. It is much better to 

 adopt this method than to pick the specimens out of a mass of 

 materials, for by sprinkling a small portion over the slate every 

 individual particle can be recognised, and the organic forms sepa- 

 rated from the inorganic, with very little trouble. A pair of 

 blunt forceps are exceedingly useful for picking up the speci- 

 mens. Of course care is necessary in using it, or injury may 

 result to the specimens ; but with caution, and a little skill, the 

 forceps may be used with great delicacy of touch, and with less 



* A convenient form of metal sieve for this purpose is described in Mackie's " Geologist," 

 Tol. i., p. 249. —T, R. J. 



