THE SHELL-LIMESTONE OF DURHAM. 137 



Beyrichice (such as B. Bussacensis) in the setting on of its ridges. 

 We also see in the figure of K, Permiana, var. Schrenkiij and 

 occasionally in some of the Durham specimens (see fig. la), a 

 tendency to the Lilobed condition of the valve, due to the sub- 

 central dorsal furrow, so characteristic of the Leperditice. The 

 oval spot, however, the hexagonal reticulation, the long stretch- 

 ing wrinkles and riblets, and the doubly and sometimes trebly 

 ridged ventral margin, are characters not found in Beyrichia, and 

 must be regarded as distinctive of a separate generic group. 



Lastly, it seems to me probable that the sunken oval spot on 

 the valves may have reference to the place of attachment of the 

 transverse muscle of the animal, and thus rej^resent the variously 

 patterned '^muscle-spots" of other bivalved Entomostraca. 



CYTHEEIM. 



Amongst the Entomostraca of the shell-limestone none are so 

 plentiful as the BairdicB. Of these there are several forms, which 

 somewhat resemble^ the Bairdia curta (M'Coy) of the lower car- 

 boniferous rocks of Ireland, and are not very dissimilar to the B. 

 suhdeltoidea (Mlinster) of the Cretaceous and Tertiary formations, 

 and of existing seas. These Permian specimens vary consider- 

 ably among themselves in outline and relative convexity. Several 

 of them were described in the " Annals of Natural History," 3rd 

 series, vol. ii., pp. 324, &c., under the specific names of B.pleheia 

 (with three varieties — elongata, Neptimi, and compressa), B. 

 ventricosa, B . Eeussiana,B. Kingii^B. inucronata, B. Schaurotliiana^ 

 B. Bermciensis, and B. rliomhoidea. 



Three other Cytheridw^ from the shell-limestone, were also de- 

 scribed at the same time as Bairdice, viz., B. reniformis^ B. Jonesi- 

 ana^ and B. truncata, which^ however, are regarded by my friend 

 Mr. Jones as more likely belonging to another sub-genus. 



Having asked Mr. T. Ii. Jones for his opinion on the arrange- 



VOL. IV. PT. II. s 



