from the Fossiliferous Limestone of Durham. 319 



for the preservation, and subsequent extraction, of microscopic 

 organisms, than this in which these Entomostraca occurred, can 

 be conceived. It has preserved the specimens which it encloses 

 so perfectly, that they almost rival the productions of recent 

 zoology. In some instances a brush will remove the investing 

 matrix, without the use of other tools. Occasionally the dust 

 occurs in a state of semi-coherency, with nodules or fragments 

 of limestone intermixed. In this state the imbedded fossils are 

 also easily extracted without much assistance from palseonto- 

 logical instruments. Some of the finest specimens ever taken 

 from the Durham Permians have been got from these cavities ; 

 and they have also yielded some of the rarest species. An ana- 

 lytical examination of the contents of some show very curious 

 results. For instance, in one I found about one hundred and 

 fifty perfect and imperfect spines of Archaocidaris Verneuiliana, 

 King, and with them but one or two fragments of the plates of 

 the same species. In another I got more than four hundred 

 dorsal valves of young individuals of Strophalosia Goldfussi, 

 Miinst., which were accompanied by only fifteen ventral valves. 

 Of course, in both cases other species were associated ; but those 

 mentioned were the most prevalent, and by their abundance and 

 peculiar mode of occurrence formed the characteristic feature of 

 the contents of each cavity. Such facts are very puzzling, and 

 would almost seem indicative of a drifting of specimens prior to 

 fossilization, — an idea which is scarcely tenable when the general 

 evidence of the whole fauna is considered. 



The majority of the specimens of Entomostraca which were 

 found belong to species of Bairdia, with scarce, but good, exam- 

 ples of a Leperditia-like species, previously noticed in the upper 

 members by Jones, and provisionally referred by him to the 

 genus Dithyrocaris of Scouler*. The Bairdia generally occurred 

 with united valves, though single valves were not uncommon. 

 Specimens of the latter species were nearly always in a detached 

 state, only three or four per cent, occurring with valves connected. 

 The valves of Bairdia possess a stronger hinge than the latter, 

 one valve overlapping the other along the dorsal margin, while 

 the valves of the other appear to have been merely united by 

 membrane. This difference in the mode of hingement seems to 

 explain their manner of occurrence. None of the specimens are 

 worn, nor show indications of drifting. The acute extremities 

 of several of the species, and the delicate margins of the detached 

 valves, never present traces of attrition ; so that it may be in- 

 ferred that these Entomostraca were original residents in the 

 area where they have become imbedded and fossilized. 



* Mon. Perm. Foss. of England, p. 64, and the remarks on this species 

 in the present paper. 



22* 



