328 Mr. J. W. Kirkby on Permian Entoniostraca 



angulated ; others more rounded, though of a subaugulated out- 

 line. The sinus of the ventral margin is scarcely indicated in 

 some specimens. The contour of the dorsal margin undergoes 

 slight variations. 



In identifying this species with B. mucronata of Dr. Reuss, I 

 have allowed a little for probable imperfection in his material, 

 or for slight modification of form which may have been induced 

 by some unknown causes. It is the first reason, however, that 

 has influenced me most ; for all the specimens of Entomostraca 

 which I have seen from the locality whence Dr. Reuss procured 

 this species are in a very unsatisfactory state, and not at all to be 

 relied on for exactness of original form ; so that it is not unlikely 

 that the specimen or specimens from which he described, and 

 which he figured, were in some measure imperfect. Should this 

 not be the case, and should his figure represent a perfect indi- 

 vidual, then our Durham form, with which I have identified it, 

 must be made a distinct species. 



Found rather commonly in the fossiliferous limestone of 

 Tunstall Hill. It is rare in the Lower Zechstein of Bleichen- 

 bach, and occurs in the same deposit at Saalfeld, Germany. 



6. Bairdia ? PI. X. figs. 12 & 12 a. 



Length ^ inch ; height ^ inch. 



Carapace subrhomboidal, attenuate, smooth. Dorsal margin 

 arched ; anterior slope moderately steep, descending one-third of 

 height ; posterior slope gradual, descending two-thirds of height. 

 Ventral margin with a central sinus, convex near extremities. 

 Anterior extremity angulate at the juncture of dorsal and ven- 

 tral margins, which almost form a right angle ; ventral curve of 

 extremity rather convex. Posterior extremity pointed, convex 

 ventrally, somewhat concave dorsally. Lateral contour very at- 

 tenuate; greatest diameter central, less than one-fifth of the length. 



I have only found a single example of this form, and it is 

 scarcely so well preserved as I could wish : and there exist some 

 doubts in my mind as to the propriety of describing it as a spe- 

 cies ; for it may be but a variety of B. mucronata, and the some- 

 what imperfect condition of the specimen rather invalidates the 

 authenticity of the specific characters which appear to distin- 

 guish it. Of this, however, there can be no certainty until other 

 and better specimens are procured. The evidence deducible 

 from this single example seems to favour the idea of its being a 

 species. Its attenuate form, and other peculiarities already 

 noticed, seem to point to specific distinction; and, relying in 

 some measure on these characters, I have thought it advisable 

 to keep it separate from any of the preceding species, and to 



