568 , R. JONES AND J. W. KIRKBY ON 
Carapace elongately subdeltoid in outline; length in type spe- 
cimen less than two and a half times the height, in others con- 
siderably more than that. Dorsal border arched (in some examples 
flatly so), with both slopes slightly concave; ventral border straight 
or faintly incurved. Anterior extremity broad, rounded below, and 
angulate above; posterior extremity produced and rostrated. Left 
valve overlaps the right moderately on both borders. Surface 
smooth. Length 55 to ;4 inch. 
Fig. 1 represents Sir R. Griftith’s original specimen (cleared from 
the matrix), from which Prof. M‘Coy described the species. Fig. 3 
is from a very beautiful example in the collection of Mr. John 
Young. Both it and fig. 6 are relatively longer, flatter in the arch- 
ing of the dorsal border, and with a more broadly rostrated anterior 
end than the type specimen. Fig. 5 represents a right valve which 
is included in this species with some doubt. The posterior extremity 
is less elevated than in other specimens, and is acutely pointed 
rather than rostrated, and it has a straight slope on the hinder 
third of the dorsal border. Still the specimen has more the cha- 
racter of curta than of any other species; we refer to it as B. curta, 
var. terebra, and notice that it is near the Permian B. rhomboidea, 
Kirkby, Trans. Tyneside N. F.-Club, 1859, vol. iv. pp. 147 & 149. 
Fig. 7 represents a variety which we name bicornis. Its dorsal 
border is elevated and flat in the middle region, with deeply exca- 
vated anterior and posterior slopes, both of which curve upward to 
form acutely angular extremities. The extremities make two strong 
convex curves, meeting at the middle of the ventral border, which 
is thus sinuated and incurved at the centre; it is rather more pro- 
tuberant anteriorly than behind. The anterior extremity is thus 
broad and trenchant, and the posterior acutely and obliquely 
rostrated. The central portion of the valves rises abruptly from 
the regions adjoining the ventral border and the ends, which are 
compressed. The lateral outline of the carapace resembles the shape 
of a Scythian bow. 
Fig. 8, var. deformis, is from an example from Steeraway, Salop, 
that has much in common with the one just noticed; but the slopes 
of its dorsal margin do not curve upward to acute points as in fig. 7, 
and the anterior third of the carapace is much more protuberant 
ventrally. In Mr. Charles Moore’s collection. 
Localities of Bairdia curta. 
In England. Carboniferous Limestone: Wyebourne, Cumberland ; 
Settle, Yorkshire; Steeraway, near Wellington, Salop. 
In Scotland. Carboniferous Limestone (Upper): Kennox Water 
(Douglas) and Wester House (Carluke), both in Lanarkshire. 
Carboniferous Limestone (Lower): Braidwood (Carluke), Brockley 
(near Lesmahagow), Shields Farm (Kast Kilbride), Calderside (South 
Shiells), all in Lanarkshire ; Causland and Fernie Hill (Gilmerton), 
in Edinburghshire, 
In the ‘Annals & Mag. N. Hist.’ ser. 1, 1847, vol. xx. p. 229, 
