BRACHIOPODA. 
137 
uncommon form in the sandy shales of the Hamilton group. The upper 
valve is elevated and obtusely conical, with its apex slightly posterior, while 
the lower or perforated valve when uncompressed 
is extremely concave, much more so on the ante¬ 
rior than on the posterior slope, the entire surface 
of the valve rising for a considerable distance into 
the cavity of the other valve. Thus the general 
contour of the pedicle-valve is precisely the re¬ 
verse of that usual in OrBICULOIDEA, while the FIG - 63 - Diagrammatic figure of Rcemerella 
grandis, showing the concavity of the 
structure of the pedicle-aperture appears to be i>edicie-vaive. 
essentially alike in both. This reversion is a sufficient basis for the separation 
of this species and all others which may be found to agree with it in this 
respect, from Orbiculoidea, and we propose therefor the sub-generic term, 
Rcemerella, sub-gen. nov. 
Type, Rcemerella grandis, Vanuxem. (See Plate IV e, figs. 29-31.) 
Orbiculoidea is not positively known as a member of faunas older than the 
Silurian. The Discina Acadica of Hartt (Dawson’s Acadian Geology, Second 
Edition, p. 644. 1868), from the St. John formation, is not now regarded as a 
brachiopod. Mr. Walcott refers it to the Gasteropod? genus, Palacm^ea?, and 
Mr. Matthew to Stenotheca, s.g., Parmorphorella. Discina ? inutilis, Hall, from 
the primordial sandstones at Mazomanie, Wisconsin, is very imperfectly known. 
The American palaeozoic formations are known to contain about forty described 
species of Orbiculoidea. The genus appears to have developed rapidly in 
species and have attained a culmination in the Middle Devonian faunas, from 
which a considerable amount of undescribed material is to be found in collec¬ 
tions. In the Menevian of Wales, Discina pileolus, Hicks, appears to be a genuine 
Orbiculoidea, and if so, may be considered as the earliest known representative 
of the genus. The D. Caerfaiensis, Hicks, from the Middle Caerfai group of St. 
Davids, is known only from its upper valve. 
