Nov., 19 IS] Structure of A gelacrinites and Streptaster 69 



peripheral rim. A . pileus may parallel the rim with the tips of 

 the rays for some distance. In A. cincinnatiensis the rays at 

 their distal ends leave the rim and point back toward the 

 center. The diagnostic character of A. holbrooki is that the 

 tips of the rays return from the peripheral rim perhaps one-third 

 of the distance toward the mouth region, dividing each inter- 

 ambulacral region at the distal edge into two parts. I have 

 never seen a small specimen of this form and see no reason why 

 it may not be an exceptionally aged A. cincin?iatiensis. . 



In all the species thus far mentioned the cover plates tend 

 to be triangular or quadrangular with a spur on the proximal 

 face of the plate between it and the next plate toward the 

 mouth on the same side. It is difficult to identify such a spur 

 always in the case of A. cincinnatiensis and I assume that 

 it may have been merged into the flange of the groove of the 

 outer row of cover plates and have been lost from the inner row 

 entirely. It probably had to do with the muscular attachments 

 of the plate. 



The genus Streptaster is characterized by covering plates 

 which are proportionately longer and more slender and which 

 interlock in a slightly different way. One specimen from the 

 Dyer collection shows a series of alternating small median plates 

 as well. If the double series of plates is normal for Streptaster 

 the inner row is lost to view in essentially all specimens found. 



PERISTOMIAL PLATES (ORAL SIDE). 



According to Zittel the characteristic number of peristomial 

 plates is four. Foerste's diagram of A. pileus shows five special 

 plates in the region of the mouth. The number of especially 

 large peristomial plates is usually three, one large, roughly 

 triangular with one angle toward the posterior inter-radial 

 space and the other two angles lying along the line between 

 the junctions of rays one, two and rays four, five, and two 

 smaller quadrangular plates opposite the large plate. These 

 quadrangular plates are in line with the rows of cover plates, 

 which make the third or anterior ray. 



There is a good deal of variation in these peristomial plates. 

 Meek's figure of A. cincinnatiensis in the Ohio Geological 

 Survey shows a modification of the pattern. The Dyer col- 

 lection has an A. pileus in which the posterior plate as shown 

 is in two parts. Figure 13, Plate III of A. austini shows a 



