ON SOME PELMATOZOA FROM CHAZY LIMESTONE OF NEW YORK IO7 



enlargement and it is therefore not figured. Under the micro- 

 scope the specimen shows the walls of the hydrospires to consist 

 of irregularly thickened and corrugated sheets so constructed 

 as to give strength with use of very little material and so secure 

 the thinness necessary for the exchange of gases with the sea 

 water. The slight bending of the inner ends of the older hydro- 

 spires in figure 2, the loss of the inner edges of several of the 

 larger and the detached and shifted edges of others, indicated 

 by arrows, all point to their membranous character and their 

 tearing during decay. 



The addition of brachioles and hydrospires while consecutive 

 was broken by periods of rest, and " growth lines " so formed 

 may clearly be seen in plate 5, figures h and i, the older deltoids 

 being easily recognized within their newer margins. 



The younger, thinner deltoids show vertical grooves on their 

 upper surfaces which lie over the hydrospire grooves below. 

 The thicker additions made to the edge of the plate by the 

 upward growth of the newer brachioles soon mask their external 

 parallelism and give rise to a series of external' ridges running 

 at right angles from the lateral plate margins. 



It would seem that young plates having but 12 hydrospires 

 could hardly have had any portion of their bases supported by 

 the bibrachials. The interbrachial plates alone would support 

 the lower edge of the deltoid and the form viewed axially would 

 be simply pentagonal, without the asteroidlike projections, or 

 as in Troostocrinus. 



The increase in width of the deltoid would be accompanied by 

 increase in length of the bibrachials. Their peculiar form is 

 thus in part brought about by the greater growth at their 

 distal ends. It may be noted that this extension has constantly 

 carried the distal end of an ambulacrum farther away from its 

 radial plate. Earlier stages in development would show that 

 closer proximity possessed by its ancestral forms and from 

 which the Eublastoidea with their notched radials took a diver- 

 gent line. 



A comparison with Codaster leads one to the conclusion that 

 the deltoids, in their origin, were true deltoids or orals. 



I may have asserted rather too positively that the hydrospires 

 lie in the grooves on the inside of the plate. In the cross-section 

 of the deltoid the boundary between the coelomic cavities and 

 the plate could not be readily made out. The outer ends of the 

 hydrospires are clearly marked by carbonized lines convex out- 



