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 pre'sents the first combination of the smooth, 

 biconvex valves with an unmodified RENSSELiERiA-CENTRONEi.LA 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 CentroneUa Julia, A. 

 Winchell, one such small, smooth species, from the Marshall group of Michigan, 



FIG. 187. FIG. 188. 



JRomingerina Julia, Winchell. 



Fig. 187. A restoration of the loop; showing the extent of the meilian pl.ite. 



Fi". 188. A profile view; showing the elevation of this plate, the double curvature of its upper margin and its fim- 

 briated edge. X4. (A. WiNCUELL.) 



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 fi-nm the Chemung- samistones at Rushfonl, N. Y., which has been iiienlifieii by 

 Williams as CentroneUa 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 verlical plate. 



