366 STUDIES IN EVOLUTION 



Homceospira sobrina sp. nov.* 



(PLATE XIX, figures 10-16.) 

 Rhynchonella Whitii Hall, in part. 



In the examination of a large number of the specimens 

 which have usually passed under the name of Camarotoechia 

 Whitii, the writers have become convinced 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 0. 

 Whitii, but in their immature condition are readily separable 

 from this species, and form, of themselves, a satisfactory and 

 well-defined developmental series. The similarity of these 

 examples with 0, 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 exter- 

 nal 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, con- 

 centric growth-lines. Internally the difference is more em- 

 phatic, as carefully prepared specimens show well-defined 

 spirals having their apices near the lateral margins, as shown 

 on Plate XIX, figure 12. While disavowing the intention of 

 describing new species as remote from the purposes of this 

 paper, the writers have, for convenience in utilizing this form 

 for their work, to which it makes no unimportant contribution, 

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



Camarotcechia sobrina, one of the more abundant of the 

 Waldron Brachiopoda, is itself subject to some variation, 

 more considerable, indeed, than that noticed in either of the 

 species Camarotcechia Whitii and C. neglecta. The material 



[* Originally referred to the genus Retzia.~\ 



