THE SHELL-LIMESTONE OF DURHAM. 149 



slopes nearly flat ; anterior extremity less acute than in 

 the ty])ical form; valves compressed laterally, the greatest 

 diameter being only a little more than half the height. 



Bairdia? Berniciensis, Kirlhy. PL IX., fig. 15; woodcut 13. 

 B.? Berniciensis. "Ann. Nat. His.," 3rd ser., vol. 

 ii., p. 330, pi. X., fig. 15. 

 Length, l-25th of an inch; height, l-55th of an inch. 

 Carapace sub-rhomboidal, moderately convex, smooth ; 

 dorsal margin convex; antero-dorsal region slightly con- 

 cave, sloping gradually one-third of height ; postero- 

 dorsal region convex, sloping abruptly one-half of height ; 

 ve?itral margin somewhat convex; anterior extremity ob- 

 liquely truncate; posterior extremity rounded or bluntly 

 pointed; lateral contour sub-lenticular, greatest diameter 

 posteriorly placed. 

 Not being able to make out the hingement of this specimen 

 (having found but one), some doubts may exist as to its really 

 belonging to Bairdia ; but, until we know more of its affinities, 

 it may be well to consider it a member of that group. 



Bairdia rhomboidea, Kirkhy. PI. X., fig. 3; woodcut 14. 



B. rliomboidea, Kirkby. " Ann. Nat. His.," 3rd 

 ser., vol. ii., p. 433, pi. XL, fig. 3. 

 Length, l-23rd of an inch; height, l-50th of an inch. 

 Carapace sub-rhomboidal , protuberant centrally, smooth ; 

 dorsal margin prominently convex, sloping gradually to 

 each extremity ; ventral margin convex, more so anteriorly 

 than posteriorly ; anterior extremity convexly sub-angulate ; 

 posterior extremity somewhat produced, bluntly pointed ; 

 lateral contour lenticular; greatest diameter central. 

 Very rare in the shell-limestone of Tunstall Hill. 



NOTES ON THE BAIRBim FROM TUNSTALL HILL, BY MR. T. RUPERT 



JONES, F.G.S. 



With regard to the majority of the specimens collected by Mr. 

 Kirkby, the valves are so well preserved that every important 

 ''eature, except the muscle-spots, can be readily distinguished. 



