1915 260 F. A. Bather — Studies in Edrioasteroidea. 



A contrasolar curve of the rays, though not constant, is certainly 

 prevalent in the Edrioasteroidea as a whole, and is regarded by 

 Dr. A. F. Foerste (1914) "as the primitive condition among species 

 with curved rays". Any explanation must, however, take into 

 account the fact that it is so generally accompanied by a solar curve 

 of the right posterior ray (V). To explain the latter feature, 

 Dr. Foerste has put forward some plausible speculations, which if 

 provided witli a more extensive foundation of observed facts, might 

 go far to solve the whole problem. 



Text-figure 1. — Diagrams to show the effect of a sloping position on the 

 course of the rays. 



A. A specimen of Agelacrinus pileus, traced from a photograph 

 published by A. F. Foerste, 1914, op. cit. , pi. ii, fig. 1. x 2 diam. 



B. A diagram showing the effect of gravity on rays still marked by 

 triradiate symmetry. 



C. Edrioaster bigsbyi placed in the same position as A and B. 

 Nat. size. 



From observations mainly on Agelacrinus p ileus, Dr. Foerste shows 

 that the theca is frequently sagged to one side, and from this he 

 infers the direction of slope of the surface (e.g. a valve of the 

 brachiopod Rajinesquina) to which the Edrioasteroid was attached. 

 If his argument be correct, then it appears that the right interradius 

 (V/IV) was generally directed upwards, so that the anus was to the 

 right of the mouth. Assuming, as is natural, that the slope of the 

 brachiopod valve was toward the direction of the prevailing currents, 

 then the anal stream would be swept away without passing over 

 either the mouth or any considerable part of the food-grooves ; 

 moreover the greatest width of the peristome would be in the line of 

 the current. So far, then, the interpretation of the facts is consistent 

 with a healthy pelmatozoic mode of life. (Text-fig. 1, a.) 



