THE WIIELL-LIMESTONE OF DURHAM. 125 



were nearly always in a detached state, only three or four per 

 cent, occurring with the valves connected. The valves of Bairdia 

 possess a stronger hinge than those of Kirkhja, one valve overlap- 

 ping the other along the dorsal margin, while the valves of the 

 other appear to have been merely united by membrane. This dif- 

 ference in the mode of hingement seems to explain their manner of 

 occurrence. None of the specimens are worn, or show indications 

 of drifting. The acute extremities of some of the species, and 

 the delicate margins of the detached valves, never present traces 

 of attrition ; so that it may be inferred that these Entomostraca 

 were original residents in the area where they became embedded 

 and fossilized. 



In the calcareous dust which contained the Entomostraca, 

 were also a number of specimens of a curious Foraminifer, ap- 

 parently a species of Miliola* Several species of Mollusca also 

 occurred, as Prodactus horridus, Spiriferina nndtipUcata, Camaro- 

 phona cru7nena,^ Crania Kirkhyi^ Monotis spehmcana, Pleuroto- 

 maria Verneidli, &c. The convex valve of a Conchifer appears 

 to have been a popular place of resort with the Bairdice, for out 

 of one I procured some dozens of individuals. J 



During the Permian period, the prevailing forms of Entomos- 

 traca seem to have belonged to two groups — to Bairdia, a sub- 

 genus of Cytliere, and to Kirkhya. Bairdia is most characteristic 

 of the Permian limestones of Western Europe; in fact, it is un- 

 known as yet in the Permian rocks of Eussia. In Germany, it 

 is represented by several varieties, and by a greater number in 

 Durham ; five varieties have been met with in the York- 

 shire Permian rocks. In the shell-limestone of Durham the 

 Entomostraca almost exclusively consist of varieties of this 

 genus, from which it may be inferred that the conditions pre- 

 vailing during the accumulation of that deposit were highly 

 suitable for their development. Bairdia seems to represent the 



* This is the same fossil as the Serptda pusilla of Geinitz, and the Spirillina pusilla of 

 Jones. 



t Late Schlotheimi. 



X In aquaria, it is not unusual to see a little host of Candonoe and Cyprides swarming 

 over and within the shell of a recently dead Paludina, or other mollusk. Possibly the 

 little marine Entomostracous scavengers were engaged in removing the dead substance of the 

 bivalve mollusk lierc referred to, and were pepulchred in its closed and buried valves. — 

 T. K. J. 



