Nov., 1918] Structure oj A gelacrinites and Streptaster Gl 



Agelacrinus: "Calyx in the form of a depressed or convex 

 disk, stemless and attached by the entire under surface. Com- 

 posed of numerous small polygonal, usually imbricating plates 

 which are perforated by fine pores, mouth surrounded by four 

 oral plates; radiating from this are five small, more or less curved 

 arms which are embedded in grooves on the outer surface and 

 are protected by a double row of covering plates. " 



Following Bather the recent classification puts A gelacrinites 

 into the family Edrioasteroidea, Billings. All evidence at hand 

 indicates that A gelacrinites differs from Edrioaster and Lepido- 

 discus in that the aboral wall contains no plates of the type 

 which Bather calls the concentric frame plates and which may 

 have been attached to the substratum. 



The Richmond Agelacrinitidce have the rays arranged in 

 one of several ways. They are either all straight as in A. 

 rectiradiatiis, all turned in a counter-clockwise or contra-solar 

 direction or else four of them are so oriented and the fifth is 

 reversed at its outer end so as to produce one inter-radial area 

 much larger than the others. 



In this larger space, which may be called the posterior 

 inter-radial space, are to be found the anal pyramid and an 

 opening which is probably the opening for the water-vascular 

 system, the hydropore. 



If the specimen be so placed that the inter-radius containing 

 the anal pyramid is toward the observer, the ray at the left is 

 known as No. 1 and the one at the right No. 5. Food-grooves 

 1 and 2 are closely related to each other and food-grooves 4 

 and 5 are also closely related to each other. Food-groove 3, 

 the anterior ray, pointing directly away from the anal inter- 

 radius is separated from the right and left pairs of rays by some 

 distance as measured on the food groove. Near the base of this 

 ray are often found three especially large plates, the peristomial 

 or mouth plates. 



The genus Streptaster Hall differs from A gelacrinites in that 

 it has proportionately larger and longer plates covering the 

 food-grooves and proportionately smaller inter-radial plates. 

 In most cases as one looks down on a specimen of this genus 

 the ends of the palisade-like covering plates are all that is visible 

 and no trace of the inter-ambulacral covering is evident. 



