THE SHELL-LIMESTONE OF DURHAM. 145 



which, in all probability, is quite distinct from the present 

 species. 



B. PLEBEIA, Var. AMYGDALINA. PI. IX., fig. 11 ; WOodcut 5. 



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



ser., vol. ii., p. 327, pi. X., fig. 11. (Not B. 



mucronata, Reuss.) 



Carapace sub-ovate ; narrow and rounded in front ; obliquely 



acute behind; antero-dorsal region gently sloping; -pos- 



tero-dorml, with a rapid slope and slight incurvature; 



ventral margin straight, with a bold curve anteriorly, and 



a slight curve upwards behind ; lateral contour shows 



compressed extremities, especially in front, and a strong 



convexity a little behind the middle. 



B. PLEBEIA, var. ELONGATA. PI. IX., fig. 4; woodcut 6. 



B. pleheia, var. elongata. " Ann. Nat. His.," 3rd 

 ser., vol. ii., p. 325, pi. X., fig. 4. 

 Carapace e\o\\^iiiQ; jwstero and antero-dorsal regions slop- 

 ing gradually; anterior extremity somewhat produced; 

 lateral contour more compressed than in B. pleheia proper. 

 The present variety is B. pleheia proper elongated and com- 

 pressed ; curtail its length and increase its width, and we have 

 the typical form of the species. 



B. pleheia, var. elongata, is a common form at Tunstall Hill ; 

 it also occurs in a magnesian limestone at Hampole, Yorkshire. 



B. PLEBEIA, var. Neptuni. pi. IX., fig. 5; woodcut 7. 



B. pleheia, var. NeptuuL " Ann. Nat. His.," 3rd 

 ser., vol. ii., p. 325, pi. X., fig. 5. 

 Var. Neptuni has an elongate carapace, with the greatest 

 convexity of dorsal margin very posteriorly situate; it 

 consequently possesses an abrupt posterior slope, and a 

 , very gentle anterior one ; its posterior extremity is 

 broadly beaked, and its latei-al contour somewhat com- 

 pressed. 

 Besides occurring at Tiiiistall Hill, I have also met with this 

 variety at Hampole, Yorkshire. 



VOL. IV. PT. II. 



