PaliBozoic Bivalved Entomostraca. 251 



hump ; the right valve is without it ; and the surface of both is 

 smootli and unornanicntcd, except that the muscle-spot is seen 

 under a lens to be neatly and faintly reticulated, but apparently 

 unaccompanied with radiated vascular nuu-kings (figs. 8 & 9). 



These specimens vary much in size. There is a fragment of an 

 individual which was larger than even the largest of the speci- 

 mens from the grey limestone of Williamsville, On the other 

 hand, there is a single left valve only i- inch long, — probably of 

 a young individual : this is broader in proportion to its length 

 than the larger individuals, being ovate in outline, and presents 

 no dorsal hump, which, from this, would appear to be acquired 

 in the adult state only. 



The differences between L. gibbera of the Arctic limestone 

 and these specimens from the United States are — the absence of 

 pittings on the surfaces of the latter, and the smallncss of the 

 dorsal hump of their left valve. There are also larger individuals 

 among these more southern specimens ; but, as the number of 

 the Arctic specimens was very limited, the exact relative size 

 cannot be regarded as fairly ascertained. From the above con- 

 siderations, I regard the specimen under notice as belonging to 

 a variety of L. gibbera. 



A thin scam of hard grey limestone, half an inch thick, on a 

 rather higher horizon than that of the black limestone just 

 referred to, has its surfaces thickly beset with badly preserved 

 valves of a Leperditia, apparently of the same variety as the last- 

 described. 



3. Leperditia Pennsrjlvanica, spec. nov. PI. X. figs. 12, 13. 



Length ~, breadth /^ inch. 



Valves very convex, mostly at the middle and somewhat ante- 

 riorly ; the posterior half of the valve sloping more gradually, and 

 broader, than the anterior, and rounded ; dorsal margin straight 

 and long. 



This species is very near to L. Balthica^ of Europe and 

 L. Arctica-\ of North America; but it is narrower and more 

 convex; its eye-spot, which is very distinct, and placed on an 

 angular escutcheon (as in L. Arctica), is rather nearer to the 

 dorsal edge, and is accompanied by greater local unevenness of 

 the surface, than in either L. Balthica or L. Arctica ; and the 

 substance of the valves is thinner than in these species. 



A specimen of greyish limestone from near Barre Forge, 



* Amials Nat. Hist. 2 scr. vol. xvii. pi. 6. figs. 1-5. 

 t Ibid. \)\. 7. fips. 1-5. 



