186 C. R. Key 68 — Carboniferous Echinodermata 



Postscript. 



As has been previously stated, the only serious source of 

 error to be feared in this method is a mistake in the whole 

 number of waves in one or more of the " intermediate stand- 

 ards." 



Notwithstanding the cumulative evidence just cited, it must 

 be admitted that such a mistake may have occurred — and the 

 numbers given in the table would require further and inde- 

 pendent confirmation before being accepted as final. 



Some months after the preceding paper was read a letter was 

 received from Professor Rowland giving numbers for the rela- 

 tive wave-lengths of sodium and mercury lines by the grating 

 method, as follows : 



The upper line repeats the means of the preceding table 

 corrected to agree with the latest of Rowland's tables by add- 

 ing 80 ooo ; the lower are Rowland's figures. 



Na x Hg Hgi Hg 3 



M. & M. 5896-157 5460-918 5790-755 4358'470 



R 5896-156 5460-94 5790-86* 4358-49 



* Marked " poor reading." 



In view of this final and almost complete confirmation these 

 results may be taken not merely to prove the feasibility of the 

 method, but as an accurate and reliable measurement of the 

 relative wave-lengths of these radiations. As the whole num- 

 ber of waves used in this work was but a tenth of the number 

 to be used in the final work — and as the relative error will be 

 diminished in this proportion, nearly, it would seem that the 

 claim made for the possible degree of accuracy attainable by 

 this method of measurement has already been justified. 



Art. XXIY. — The Carboniferous Eehinodermata of the Mis- 

 sissippi Basin • by Charles R. Keyes. 



During the deposition of the Lower Carboniferous rocks, life 

 throughout the interior of North America was remarkable for 

 the immense development and expansion of piscine and echino- 

 dermatous types ; and among the latter especially, for the cul- 

 mination of crinoidal and Mastoidal forms. Not only was the 

 development of the Crinoidea phenomenal in the number of 

 species, but the extensive numerical representation of individ- 

 uals was most astonishing. So prolific was crinoidal life at 

 this period that the disjointed skeletal remains form great beds 

 of what may be appropriately denominated a crinoidal breccia ; 

 which, however, is not always compact, but frequently full of 



