112 NEW YORK STATE MUSEUM 



end rests against one or more small plates apparently forming 

 a double row down the middle o£ the edge of the deltoid and 

 just inside of the apparently single kidney-shaped foundation 

 plates of the brachioles. These small plates, with probably 

 some others, serve to floor a rather large brachiolar cavity which 

 is represented in figure 2 by shading its boundaries. The sec- 

 tion, which is rather thick, admits much light over this area and 

 thus suggests a series of connected brood chambers. The 

 boundary plates of this cavity require further study. The inner 

 end of an adambulacrum has one face against a covering plate, 

 a middle concave portion bounding nearly a fourth of a circular 

 food groove, and an inner or lower face that abuts against the 

 opposite row of adambulacrals. 



Between each plate and its neighbor in the same row there 

 are two openings, one into the food groove along the line of 

 juncture of the upper, inner faces of the plates and one into a 

 hydrospire along the line of juncture at their outer or deltoid 

 edges. The plate is grooved from the middle of the longer 

 concave upper surface toward the food pore on one side and 

 again from the same middle portion toward the hydropore ( ?) 

 on the other side. This gives the appearance of a little twist 

 to this outer long edge of the plate and shows that the brachiolar 

 chambers along the side of an ambulacrum were probably con- 

 nected with each other. The older plates retained the power 

 of extension of their stereom and the upper figure of plate i will 

 show that the older became the larger and very materially wid- 

 ened the ambulacrum. These plates rather strongly suggest 

 the ambulacral plates of Asterias. 



There is no trace of a lancet plate and perhaps the question 

 of homogenesis of bibrachials and lancet plates is worth con- 

 sidering. Our species has little to offer, but its bibrachials 

 partly separate the deltoids and reach the primary meristem of 

 a ray at one end, while the other abuts against the apex of a 

 radial. This is closely the position of the lancet plate in 

 Codaster. The lancet plate of Eleutherocrinus with its seem- 

 ingly double oral ends would suggest that possibly the primitive 

 lancet plate was double. 



Covering plates. The covering plates of an ambulacrum are 

 remarkably large and heavy. Each is as wide as the adambula- 

 cral directly over which it rests and its thickness is extraordinary. 

 The outer or side surfaces of a row are slightly concave and very 

 smooth; against these surfaces rests a portion of the brachioles. 



