In Davidson's Monograph of the British Carboniferous Brachiopoda (1857), p. 60, he gives the following specific characters of Spirifera glabra:—“Very variable in shape and proportions; transversely oval, rarely as long or longer than wide. Valves almost equally convex, with a mesial elevation or fold in the dorsal, and a sinus in the ventral valve. Hinge-line much shorter than the greatest width of the shell; cardinal angles rounded; beaks rather approximate, that of the larger or ventral valve prominent, incurved, and of moderate dimensions. A hinge-area in the dorsal valve, that of the ventral one triangular and of moderate dimensions, with its lateral margins more or less sharply defined; fissure partially covered by a pseudo-deltidium. The mesial fold in the dorsal valve is either slightly and evenly convex, rising gradually from the lateral portions of the valve, or abruptly elevated, with a longitudinal depression along its middle, which is also at times reproduced in the sinus of the ventral one. The spiral appendages are large, and occupy the greater portion of the interior of the shell. Surface of valves in general smooth, but sometimes a few obscure rounded ribs may be observed on their lateral portions”