THE SHELL LIMESTONE OF DURHAM. 135 



ture, which is indicated in Richter's figure (" Zeitschrift," vol. 

 vii,, pi., 26, fig. 2.), I could not find clear evidence on the 

 specimens which I possess from Byers' Quarry, although a trace 

 of reticulation on one valve, and a slight subcentral circular 

 mark on the casts of others, strengthened my belief that they 

 might belong to the peculiar genus which comprehends the so- 

 called Cifthere Roessleri. 



Mr. Kirkby's specimens from Tunstall Hill, occurring under 

 the circumstances so well described by him (see above), enabled 

 him to set these peculiar Bei/ncJua-like forms again before palae- 

 ontologists, and this time with full and clear descriptions, accom- 

 panied by several sketches ("Annals of Natural History," 3 ser., 

 vol. ii., pi. 11, figs. 5-13). Following Mr. Kirkby's suggestions, 

 assisted by his notes, and carefully studying his numerous speci- 

 mens, I am now enabled to show them in a lithogra23h executed 

 by Mr. G. West, with their details elaborated, and in comparison 

 both with the German forms and with the specimens from Byers' 

 Quarry. 



Some of the features which I have lately recognized in Mr. 

 Kirkby's s|)ecimens are alluded to above in his revised descrip- 

 tion of the species. I need only add the following remarks. 



The sculpture of the surface of the valves is subject to con- 

 siderable variation as to its distinctness. In the specimens which 

 I have seen from the Zechstein of Saalfield and Gera, it is very 

 distinct and uniform ; the subcentral oval spot is well defined ; and 

 the inner border of the innermost of the ventral ridges is strongly 

 marked by a line of small roundish pits (figs. 8a and 8b ; shown 

 also by Richter). A faint trace of this linear punctation I have 

 lately found in a specimen from Byers' Quarry. Not unfre- 

 quently the longitudinal boundaries of the hexagonal pits, on 

 the surface of the valves, are strengthened (fig. 2a\ and occa- 

 sionally exaggerated (figs. 4 and 6), in which case we get the 

 ridged variety, K. Permiana, var. glypta, which I named as a 

 species in the " Monog. Perm. Foss." Sometimes a single ridge 

 only is developed on the valve, as in fig. ha. Reuss's C. Roessleri 

 (fig. 9) is evidently one of those ridged varieties, and may be 

 distinguished as K. Permiana, var. Roessleri. Richter's C. 



