312 STUDIES IN EVOLUTION 



form from mature individuals down to as young and small 

 specimens as could be found. Abnormal examples, also, 

 were reserved and grouped with them. It was the intention 

 of the writers to accompany this memoir with photographic 

 illustrations of these series, representing each species here 

 discussed; but it has not been found wholly feasible, and the 

 illustrations are largely restricted to the presentation of the 

 immature and adult conditions of growth, with the exception 

 of the several series which are given on Plate XXII. 



The product obtained from washing the slabs was pre- 

 served and passed through sieves to assort the material into 

 different grades of fineness. It was found that these wash- 

 ings contained a great number of partially developed shells, 

 and it is from them that the extremely young brachiopods 

 treated of in the present paper have been derived. The 

 writers have carefully examined all the residue of these 

 washings, and have picked out about fifty thousand speci- 

 mens, most of which are less than five millimetres in length, 

 and many have a length of not more than one millimetre. 

 After all the imperfect and badly preserved individuals were 

 rejected, there still remained more than fifteen thousand 

 inchoate individuals. 



The sediments at Waldron consist of fine calcareous shales 

 weathering into clays. A stratum of Niagara limestone over- 

 lies the shales at this locality, but none of the fossils derived 

 from this limestone have been used in the preparation of the 

 present paper, and so far as known, it has a comparatively 

 different fauna and does not furnish such material as is here 

 described. The calcareous matter in the shales consists 

 almost entirely of fossils and fragments of fossils, principally 

 branches of corals and bryozoa, segments of crinoid columns, 

 and broken crinoid plates. The Brachiopoda are all calcare- 

 ous, and the original shell structure is more or less preserved, 

 depending upon the absence or presence of pyrite. 



The occurrence in such great numbers of immature shells 

 in these deposits may be explained by the luxuriant fauna 

 which flourished in this Niagara basin, by the quiet seas of 



