THE SHELL-LIMESTONE OF DURHAM. 143 



" cheese-knife-like form," which holds good, more or less, in all 

 the Permian varieties. It is the marked difference of this fea- 

 ture from its equivalent in B. curta, upon which Mr. Jones and 

 myself chiefly rely, in considering B. pleheia to be a distinct 

 species. 



B. pleheia occurs plentifully in some portions of the shell- 

 limestone at Tunstall Hill, also rarely at Humbleton Hill, and 

 not unfrequently in the upper limestone at Byers' Quarry. 



It is found in the Unter Zechstein at Bleichenbach, Selters, 

 Gera, and near Saalfeld,* in Germany. 



B. PLEBEiA, var. CAUDATA. PL IX., figs. 10, 9, and 12; and 

 woodcuts 2, 3, and 4. 

 B. mucronata, Kirkby. " Ann. Nat. His." 3rd ser., 

 vol. ii.,pp. 327-328, pi. X., figs. 9-10. (Not B. 

 mucronata, Reuss.) 

 Length, l-20th of an inch; height, l-45th of an inch. 

 Carapaee sub-cuneiform ; antero-dorsal region sloping more 

 gradually than in pleheia proper, and forming a sharp 

 angle, with antero-ventral curve, which forms a bold, 

 convex sweep to the ventral margin; posterior extremity 

 produced, acutely pointed, and curving upwards ; lateral 

 contour lenticular, having the extremities more acute 

 than in type specimens. 

 Var. caudata exhibits the broad cheese-knife-like antero- 

 ventral curve in its greatest development. The length and 

 acuteness of its posterior extremity is also greater than in 

 any other variety of this species. The peculiarity of these 

 features give to it a delicacy of outline which is unequalled 

 among the Permian Entomostraca. 



In classing this form as a variety of B. pleheia, I of course 

 cancel my former identification of it with B. mucronata, Reuss, 



Dr. Richter, in the " Zeitschr, deutsch. geol. Gesell.," 1855, figured and described some 

 varieties of B. plebeia, namely, B. drupacea (pi. xxvi., figs. 10-11); B. geinitziana (fig. 

 12); B. curta (figs. 13-15). On Mr. Jones addressing Dr. Ricliter respecting tlie speci- 

 mens found at Saalfeld, lie was immediately favoured by a suite of specimens and a letter 

 from Dr. Richter, who stated his opinion that his B. drupacea, and B. geinitziana, were 

 certainly varieties of what he termed B. curta. The last appears to be a variety of B. 

 plebeia. 



