THE OSTRACODOUS GENUS BAIRDIA. 569 
&e., Prof. M‘Coy, writing of the Carboniferous Entomostraca of 
Australia, refers to B. curta as occurring in the Dunvegan shales, 
together with a form which he recognizes as “ Cythere impressa,” 
M‘Coy (Beyrichia?): see Ann. N. H. ser. 3, vol. xviii. p. 44, &e. 
2, BarrpIA PLEBEIA, Reuss. Plate XXVIII. figs. 9-19. 
Be 
Bairdia plebeia, Reuss, 1854, Jahresbericht wetterau. Gesellsch. 
1854, p. 67, fig. 5. 
B. Geinitziana, Richter, 1855, Zeitsch. deut. geol. Ges. vol. vil. 
p- 530, pl. 26. fig. 12. 
B. plebeia, Kirkby, 1858, Annals Nat. Hist. ser. 3, vol. ii. p. 324, 
pl. x. figs. 1-7. 
B. plebeia, Kirkby & Jones, 1859, Trans. Tyneside Nat. Field- 
Club, vol. iv. pp. 141-146, pl. ix. fig. 7. 
Cythere (Bairdia) plebeia, Kirkby, 1861, Quart. Journ. Geol. Soc. 
vol. xvii. p. 308. 
Cythere (Bairdia) plebeia, Kirkby, 1862, Annals of Nat. Hist. 
ser. 3, vol. x. p. 203, pl. iv. figs. 5-10. 
<— Cythere plebeia, K. E. Schmid, 1867, Neues Jahrbuch, &c. 1867, 
p- 581, pl. vi. fig. 26 (not figs. 1-25 & 27-45; these are all grouped 
together at pp. 582 & 588). 
Bairdia plebeia, Jones & Kirkby, 1875, Ann. Nat. Hist. ser. 4, 
yol. xv. p. 56, pl. vi. figs. 6, 7. 
B. plebeia, as a Permian species, has been described by both 
Reuss and ourselves; but we will briefly notice its leading features 
from Carboniferous specimens. 
Subdeltoid in outline, with the valves convex except at the ends, 
which are compressed; length a little more than twice the height. 
Dorsal border arched, with the posterior slope always concave, and 
the anterior slope occasionally so; ventral border straight or slightly 
incurved. Anterior extremity rounded, most prominent above; 
posterior extremity rostrated. The right valve strongly overlapped 
by the left on the dorsal border, and on the centre of the ventral 
border. Surface smooth; rarely pitted. Length 3 to +4 inch. 
Carboniferous specimens of this species present many variations of 
form. These variations mainly consist of increase in relative length, 
of sinuation or non-sinuation of the anterior slope, of the tendency 
of the anterior extremity to become subangulated, and proportionate 
length and rostration of the posterior extremity. 
Fig. 15 is from a Craigenglen specimen, showing a coarsely 
pitted surface, in which character it resembles B. ampla; never- 
theless the general form of the carapace is that of plebeia. 
Figs. 16-18 show a much inflated form from the Carboniferous 
Limestone of Backwell, near Bristol. The lateral contour and end 
view of this specimen differ considerably from those of ordinary 
B, plebeia. 
B. plebeia appears to have been the prevailing form of the genus 
during the Upper Paleozoic periods. The following Localities are 
some of those where it has been found :— 
