2T0 



NEW YORK STATE MUSEUM 



interbrachials was lost but structures below were revealed. There 

 is evidence here that the hydrospires descended internally to a posi- 

 tion below the base of the deltoid, thus further increasing the 

 respiratory area. 



In figure i of the text we have presented a part of the lower edge 

 of a deltoid xio. The inner surface is shown and from its 



-7-! 



-W-^ 





l^ft^ 



>. ;f^-. 



;IIL,. i^k. 



\ ^ -V 







^^^^B i< P * 



^^#3fe >^ 





Fig. I From a photomicrograph of the under surface of the deltoid figured in Museum Bulle- 

 tin 107. plate 5 at o. xio. Add ten each to the number here used and they will correspond 

 with those used in the former reproduction. 



study we may conclude that with the downward extension of the 

 plate it often happened that two hydrospire exits were merged into 

 one. The grooves here marked 7 and 8 find only one exit. The 

 same is also true of pairs 10 and 11, 13 and 14, 16 and 17. Hydro- 

 spires numbered 21 and 22 appear to have had the choice of either 

 their own opening or that of their neighbor. Anastomosis of these 

 sheets would hardly be of enough advantage to become the subject 

 of natural selection as the stoppage of any one exit would not inter- 

 fere either with the food-getting capacity or with the waterfiow 

 down its own brachiole. Such stoppage would only mean a slightly 

 faster flow through the numerous other hydrospires, attached to the 

 under surface of half a deltoid. With a more primitive form in 

 which there was no anastomosis of streams above the hydrospire 

 pores such anastomoses below them might prove of great value and 

 ultimately lead to structures like the hydrospires of the Blastoidea. 

 In Canadian Organic Remains, Dec. IV, p. 21, Billings said of 

 the stem that it was " round, with an alimentary canal so small 

 that often detached joints seem to have no central perforation 

 . . . the flat faces of the separate joints exhibit strong radiating 



