SILURIAN BRACHIOPODA 61 



must, however, be regarded as largely due to magnification, the distance 

 between successive coils being actually not so great as the distance 

 between the apical turns of the ribbon in the mature spiral. 



RETZIA SOBRINA, sp. nov. 



PLATE V, FIGS. 10-16 

 Rliynchonella ivhitii, Hall, in part. 



In the examination of a large number of the specimens which have usu- 

 ally passed under the name of Rhyncho.nella whitii, we have become con- 

 vinced, that, aside from the individuals which agree with the types and the 

 description of the species, there is a series of shells, which, in the mature 

 state, may be readily confounded with immature stages of R. whitii, but in 

 their immature condition are readily separable from this species, and form, of 

 themselves, a satisfactory and well-defined developmental series. The simi- 

 larity of these examples with R. whitii is found in the general outline, the 

 strong, simple plications approximately the same in number, and the usual 

 two plications on the median fold. The external differences, however, 

 in the new species, are these : the plications on the fold may be one or 

 three, and whatever their number, the fold is always depressed, in most 

 instances even to obsolescence, and the plications upon it are low and often 

 faint. The foramen, also, is circular in maturity, with perfectly developed 

 deltidial plates, and the surface of the valves usually conspicuously marked 

 by fine, crowded, concentric growth-lines. Internally, the difference is more 

 emphatic, as carefully prepared specimens show well-defined spirals having 

 their apices near the lateral margins, as shown on plate 5, figure 12. 

 While disavowing the intention of describing new species as remote from 

 the purposes of this paper, we have, for convenience's sake in utilizing this 

 form for our work, to which it makes no unimportant contribution, desig- 

 nated it as above, as no doubt exists of its specific value. 



Retzia sobrina, one of the more abundant of the Waldron brachiopoda, 

 is itself subject to some variation, more considerable indeed, than that no- 



