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 i\iQ i^io.m.lii^sv^mm^tic&s^ve otEmmeraia 

 iv-i.j>.- <^i ^ii>^i, v^ , ^TOjiffis, showing the concavity of the 



structure of the pedicle-aperture appears to be iiedicie-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, 



R(EMERELLA, Sub-gCU. nOV. 



Type, Rcumerella grandis, Vanuxem. (See Plate IV e, figs. 29-31.) 



Orbiculoidea is not positively known as a member of faunas older than the 

 Sikirian. 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^a?, and 

 Mr. Matthew to Stenotheca, s.g., Parmobphorella. Dixcina ? 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. 



