BRACHIOPODA. 
271 
or departure in the variations of this type of structure as to require a separate 
designation. The name Selenella is therefore introduced for the subgeneric 
type, its representative species being Selenella gracilis, sp. nov. 
As already observed, this shell presents the first combination of the smooth, 
biconvex valves with an unmodified Renssel^ria-Centronella brachidium. 
From our experience such a simple combination could not be of long contin¬ 
uance, and thus far we have no evidence of its subsequent appearance. 
Biconvex and smooth-valved centronellids do occur, however, at a much later 
period and after the close of the Devonian, but these have undergone a very 
material modification in the form of the brachidium. In Centronella Julia, A. 
Winchell, one such small, smooth species, from the Marshall group of Michigan, 
Fig. 187. 
Jtomingerina Julia, Winchell. 
Fig. 188. 
Fig. 187. A restoration of the loop; showing the extent of the median plate. 
Fig. 188. A profile view; showing the elevation of this plate, the double curvature of its upper margin and its fim¬ 
briated edge. X4. (A. Winchell.) 
the median ridge on the anterior plate of the brachidium is elevated into a 
conspicuous vertical lamella, extended both anteriorly and posteriorly, being in 
fact a double plate produced by the abrupt deflection of each lateral branch of 
the brachidium near the median line; union taking place along the upper 
edge, which almost reaches the inner surface of the pedicle-valve. Professor 
Alexander Winchell, who was the first to demonstrate this structure, adds in 
regard to this feature ; * 
^ The little species from the Chemung’ sandstones at Riishford, N. Y., which has been identilied by 
Williams as Centronella Julia (Bull. U. S. Geol. Survey, No. 41, p. 56. 1887), has the brachidium of similar 
structure though with a less ante-posterior extension of the vertical plate. 
