432 Mr. J. W. Kirkby on Permian Entomostraea 



XL VI. — On Permian Entomostraea from the Fossiliferous Lime- 

 stone of Durham. By J. W. Kirkby. 



[With a Plate.] 



[Concluded from page 330.] 



Bairdia Jonesiana, n. sp. Plate XI. figs. 1, 2, & 2 a. 



Syn. B. gracilis, M'Coy, Jones, Mon. Perm. Foss. p. 63. tab. 18. fig. 7- 

 B. gracilis, M'Coy, Reuss, Jahres. Wetterau. Gesell. 1 85 1-1853, p. 65. 

 B. gracilis, M'Coy, Richter, Zeitschr. Deutseh. Geol. Gesells. 1855, 

 vol. vii. p, 530. tab. 26. figs. 16 & 17- 



Length 2V ii^ch ; height jj inch. 



Carapace reniform, convex, smooth. Dorsal margin regularly 

 arched ; anterior slope gradual, convex, descending one-half of 

 the height ; posterior slope more abrupt, convex, descending five- 

 sixths of the height. Ventral margin sinuate centrally, convex 

 near the extremities. Anterior extremity bluntly rounded. Pos- 

 terior extremity obtusely pointed. Lateral contour compressed 

 oval, the posterior end the most acute ; greatest diameter central, 

 one-third of the length. Flange of left ventral margin large, 

 being fully one-third of the length. 



The chief variation of form shown by this species is in the 

 dorsal margin, which is more prominent in some specimens than 

 in the generality. Fig. 2. PI. XI. illustrates this variety. In 

 no instance have I met with examples so attenuate as the one 

 from Byers' Quarry, figured by Jones. 



It resembles in many respects the Cy there Geinitziana^ of 

 Jones, which appears to be a closely related form. The dorsal 

 margin of the latter, however, is more fiatly convex, its posterior 

 extremity is more acutely pointed, and it is medianly placed, and 

 the sinus of its ventral margin is deeper than the same feature 

 in B. Jonesiana ; its lateral contour is peculiar also, being ovate, 

 while that of the other is a flattened oval, with rather acute 

 extremities. In these particulars it differs from the present 

 species ; and I allow them specific value. 



There can be little doubt as to the identity of this species 

 with Mr. Jones's specimen from Byers' Quarry, which he iden- 

 tified with the Bairdia gracilis f of M'Coy. A single cast, much 

 worn, was the only material which Mr. Jones possessed ; so that 

 some difference might be expected between his figure and more 

 finely preserved specimens of the same species, even had it really 

 belonged to B. gracilis, — an idea, however, which is not main- 

 tainable when the Professor's description and figures are com- 

 ])ared with ])erfect specimens of the Permian species. The de- 



* Prof. King's Mon. Perm. Fossils, p. 62. tab. 18. fig. 1 a, h, c. 



t Prof. M'Coy's Syn. Char. Carbon. Foss. heland, p. 165. pi. 23. fig. 7- 



