from the Fossiliferous Limestone of Durham. 325 



posterior extremity much shorter, and not so acute as the same 

 member in perfect examples of B. plebeia ; the anterior extremity 

 is also angulate dorsally, — a character unknown to the latter 

 species, whose anterior extremity, though sometimes a little 

 produced, is always more or less rounded. These characters, 

 with a slight sinuosity in the ventral margin, are the only points 

 in which it differs from the present species ; and as the speci- 

 mens from which they were obtained were, as above stated, casts, 

 I think they may be assumed to be accidental, and of little spe- 

 cific value. This being the case, the identification with B. curta 

 is probably erroneous; for, according to M'Coy's description 

 and figure of that species, its general form is more elongate, and 

 its extremities are more acutely rostrated, especially the anterior, 

 than those of plebeia ; it also possesses a deep sinus in its ventral 

 margin, and shows a marked difference in lateral contour, being 

 subcuneiform, while the other is lenticular. It has other pecu- 

 liarities ; but those mentioned appear sufficient to demonstrate 

 its being specifically distinct from B. plebeia. 



B. plebeia occurred very plentifully along with the other En- 

 tomostraca found in the fossiliferous limestone at Tunstall Hill, 

 also rarely at Humbleton Hill, and not unfrequently in the 

 upper beds at Byers' Quarry. 



It is found in the Lower Zechstein at Bleichenbach, Selters, 

 and probably at Saalfeld in Germany. 



Var. elongata. Plate X. figs. 4 & 4 a. 



Length y 1 ^ inch ; height -^j inch. 



This is a more elongate form than the preceding. Its pos- 

 terior and anterior slopes are more gradual, and its anterior ex- 

 tremity is more produced and more compressed laterally than 

 in that form. 



Var. compressa. PI. X. figs. 7 & 7 a. 



Convexity of dorsal margin regular; sinus of posterior slope 

 nearly obsolete ; posterior extremity less acute than the typical 

 form. Valves compressed laterally, the greatest diameter being 

 only a little more than half the height; position of greatest 

 diameter in posterior half. 



Var. Neptuni. PI. X. figs. 5 & 5 a. 



The form to which I have attached the name Neptuni shows 

 several peculiarities, which might almost be considered of specific 

 importance had they been constant ; but when a series of spe- 

 cimens is examined, it is seen to approach, by gradations, the 

 typical forms of B. plebeia, so that it seems necessary to include 

 it along with other varieties of that species. 



