Nov., 1918] Structure of A gelacrinites and Streptaster 73 



In A . aiistini var lawshe there are in the adults nine flooring 

 plates after the substomial plate in each ray. Figure 41, 

 Plate VII, shows most of these flooring plates in one ray or 

 another. In this species there are about three pairs of cover 

 plates for two flooring plates. 



In Streptaster we have the primitive condition, one pair 

 of cover plates standing on a single floor plate. This will 

 probably hold good in the case of A. rectiradiatus when the 

 flooring plates of this species are known. 



In developing forms of A . austini var lawshe to be considered 

 later it is quite clear that a flooring plate of the adult is made 

 from two primitive flooring plates a proximal and a distal, fused. 

 The crowding of the cover plates toward alternation and 

 hence toward doubling already considered is a character which 

 is correlated with this fusion of primitive flooring plates. 



ABORAL ENDS OF COVER PLATES. 



Here again the material from Lawshe gives some precise 

 information. The condition of cover plates in Miller and 

 Faber's specimen of A. pileiis as described by Foerste is not 

 common either for the specimens of A. pileus at hand or for 

 A. austini either from Dutch Creek, Cowan's Creek or Lawshe. 

 In the Miller and Faber specimen cover plates are described 

 with a long basal process extending away from the ambulacral 

 groove and under the edge of the inter-ambulacral plates. 



In the Lawshe specimens the aboral end of cover plates 

 is shown in many cases where the floor plate of the ray has 

 slipped. The plate ends rather squarely with tvv^o enlarge- 

 ments one the point of articulation with the flooring plate and 

 one more oral in position, which must have been for muscular 

 attachment. It is this enlargement or a point just above it 

 which must be prolonged as the basal extensions of the lateral 

 covering plates in A. pileus Miller and Faber and in A. cin- 

 cinnatiensis. A few of the plates on A. rectiradiatus show an 

 extension somewhat similar to these. Might it not be that in 

 aged specimens there was a deposit in the tissues attached 

 to these plates so as to produce these basal extensions? 



Between the enlargements on the bases of two adjoining 

 cover plates in A. austini var lawshe is left a small circular 

 opening which would connect between the radial food groove 

 and the visceral cavity, Figure 43 A, Plate VII. These openings 



