THE SHELL-LIMESTONE OF DURHAM. 151 



M'Coy, The differences observed between tlie specimens of 

 Reuss and Kirkby, on tlie one hand, and the figures and descrip- 

 tion of Professor M'Ooy, appeared to be of less importance than 

 what was necessary to constitute specific distinction. A short 

 time ago I received, by Sir R. Griffith's kindness, the original 

 specimen of ^. curta; and, when I had carefully worked it out 

 from its matrix, I saw that the exact outline of the species was 

 not represented in M' Coy's figure, and that the points of differ- 

 ence were quite sufficient to warrant B. curta and B. pleheia 

 being considered distinct species. These differences I have 

 pointed out to Mr. Kirkby, who mentions them in his remarks 

 on B. plebeia and its varieties. 



The shapes of figs. 8 and 7 (woodcuts 11 and 12) seem to be 

 less readily traced into the contours of any of the varieties of 

 B. pleheia, but approximate to each other ; and, therefore, I 

 imagine that they may belong to a distinct species. The lateral 

 contour in these two varieties equally show but a slight ventral 

 overlap. Fig. 15 (woodcut 13) is a peculiar form, almost ellip- 

 tical ; it appears, nevertheless, to be a Bairdia; and the same 

 may be said of fig. 3, plate X. (woodcut 14). 



Cythere {Cytherideis) Jonesiana, KirU))/. PI. X., figs. 1, 2. 



Bairdia gracilis, Reuss. " Jahres. Wetterau Gesell., 



1854," p. 65, fig. 3 (fig. 2?). 

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

 vol. ii., p. 432, pi. XL, figs. 1, 2. 

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

 Carapace reniform, convex, smooth ; dorsal margin regu- 

 larly arched; antero-dorsal region convex, gradually slop- 

 ing to anterior extremity ; postero-dorsal region convex, 

 abruptly sloping to posterior extremity ; ventral margin 

 sinuate centrally, convex near extremities; anterior ex- 

 tremity bluntly rounded ; posterior extremity obtusely 

 pointed; lateral contour compressed oval, the posterior 

 end most acute; greatest diameter central, one third of the 

 length ; flange of left ventral margin large, being fully 

 one -third of the length. 



