Nov., 1918] Structure of A gel acrinites afid Streptaster 67 



These may be the specific points of attachment of the muscles 

 of the muscular wall in which or on which the plates were 

 imbedded. 



COVER PLATES. 



There -are five "arms" on these animals, either flush with 

 the general surface or elevated above it, depending on the 

 species and also on the conditions of fossilization. These arms 

 or rays are the external evidences of the food grooves, the 

 device for obtaining more food than a simple mouth opening 

 would be able to furnish the animal. 



These food grooves are primarily trimeric in their arrange- 

 ment, one extending to the left of the anal pyramid, one to 

 the right of this pyramid and one directly away from it or in 

 the anterior direction. All Edrioasteroidea, as far as I can learn, 

 have these right and left extensions modified by dichotomous 

 branching so that there are five rays and parts of the food 

 groove instead of three. 



Beginning with the left ray by the anal pyramid as 1, these 

 rays are numbered clock-wise around the disc to 5 which is the 

 reversed ray on the right of the anal pyramid except in some 

 Streptasters. Number 3 is therefore the anterior ray. 



In A. cinci?inatie?isis, A. holbrooki and A. austini the cover 

 plates are arranged on each ray in two double rows while in 

 A. pileus, A. rectiradiattis and others the cover plates seem 

 to arch over the food groove as single pairs of plates. One 

 can select a series of stages in the crowding of the cover plates. 



Taking A. rectiradiatus as the simplest form, since in this 

 species there is no complication with curving rays, we find 

 cover plates whose free ends interlock or alternate along the mid- 

 line of the groove. Diagrams of this arrangement are shown 

 on Plate IX, Figures 54 and 56. There is usually a projection 

 on the side of the plate nearest the mouth and the plate as 

 seen from the side is heavy. Fig. 49, Plate VIII. 



A. pileus as described by Hall and by Foerste and as far 

 as my specimens give evidence, has but the two covering plates, 

 one row on each side arching over the food groove. 



A. austini, according to the description of the species as 

 given by Foerste, shows besides the lateral covering plates the 

 central or median series of covering plates, of which "a greater 

 length frequently is exposed than in the case of any other known 

 species." See Plate III, Figure 14, Plate VIII, Figure 50A. 



