THE OSTRACODOUS GENUS BAIRDIA. 567 
The Carboniferous (and Permian) forms are, like many others, 
usually more or less subdeltoidal in the general shape of the carapace, 
with the posterior end the smallest, and often pointed or rostrate. 
Other forms are subcylindrical, and others subovate, in outline. 
The left valve always overlaps the right along both margins, but 
most strongly and regularly on the dorsal margin. The surface is 
usually smooth, though occasionally pitted. In some examples the 
muscle-spot is seen; it consists of a congeries of small raised spots 
(seen as depressions on the internal casts), circularly arranged 
within a faintly sunken area, 
1. Batrpra curta, M‘Coy. Plate XXVIII. figs. 1-8. 
Bairdia curtus (only partly exposed in its matrix *), M‘Coy, 1844, 
Synopsis Carboniferous Limestone Fossils of Ireland, p. 164, pl. xxiii. 
fig. 6. 
Cythere (Bairdia) curta, Jones, 1849, in King’s Monograph of 
Permian Fossils, p. 61, pl. xviii. fig. 3. 
Bairdia curta (freed from its matrix), Jones, 1870, Monthly Micro- 
scopical Journal, vol. iv. p. 185, pl. lxi. fig. 1. 
After some oscillation of opinion, we think it best to look upon 
B. curta as distinct from B. plebera. We discussed this question in 
1859, when describing some Permian Bairdie +; and we returned 
to it in 1866, when we were inclined to look upon the two forms as 
belonging to one speciest. But after further examination, with 
the help of additional material, it appears to be least objectionable to 
consider those specimens which have the anterior extremity angu- 
lated above as belonging to B. curta, and the specimens with the 
same extremity rounded both above and below as belonging to 
B. plebeia. In coming to this decision we are aware that the most 
is made of small differences, and that the existence of intermediate 
forms is in a measure ignored. But this is only what we have had 
to do in studying the Carboniferous Bairdie generally, and to some 
extent among the Ostracoda from other formations. The longer 
we investigate, and the more specimens we examine, the greater 
difficulty do we find in grouping the various forms and varieties into 
species. Doubtless had we the animal, as well as the carapace that 
enclosed it, to assist us in determining the species, important differ- 
ences might be seen, which are only hinted at in the materials pre- 
served to us§. And this possibility has been allowed to have some 
weight in the present instance as well as in others. We describe 
the species under notice as follows :— 
* See ‘ Transact. Tyneside Field-Club,’ vol. iv. p. 151; and Ann. & Mag. N. H. 
ser. 3, vol. xviii. p. 42. 
+ Trans. Tyneside Field-Olub, 1859, vol. iv. p. 150. 
t Ann. & Mag. Nat. Hist. 1866, ser. 3, vol. xviii. p. 42. 
§ Not only specific, but even generic differences would probably be shown 
by the soft parts, were they preserved. Carapaces which we are obliged to 
refer to the one genus, Bairdia, may possibly belong to such different groups 
as Bythocythere, Macrocypris, Paracypris, and Cytherura, as defined by the 
researches of G. 8. Brady, G. O. Sars, and others. 
