u 



AESIOOYSTITES PRISCUS, «. sp. 



Plate II, Fig. 10, summit view with the outer plates of the arms 

 sillcified and showing the position of the anal or ovarian 

 piivamld, Fig. 11, .summit view with, the arm plates 

 removed and disclosing tlu' a rni furrows, the cen- 

 tral orifice and the position of the anal or 

 ovarian pyramid; Fig. 12, lateral 

 view from which the external 

 plates of the arms liave 

 been partly removed 

 and showing the 

 interbrachial 

 plates. 



The specimens upon which this genus and species are founded 

 are gilicified and the sutures between the plates more or less de- 

 stroyed. The under side of all of them is broken in or damaged 

 so that a full knowledge of it cannot be obtained. 



The general form is he.nisplierical, with truncated margin 

 rounded and radial ridges elevated and angular. It is com- 

 posed of polygonal and non-imbricating plates, the largest of 

 which are in the interbrachial areas. The iuterbracliial areas are 

 distinctly defined on the convex side and the plates curve over 

 the inferior margin and on the truncated side without interrup- 

 tion. The arms are very large, highly convex, and angular on 

 the summit. They radiate from the center in such manner as to 

 leave one interradial area larger than the others, with an arm di- 

 rectly opposite, and this lengthens the central arm elevation 

 toward the two arms upon each side. The arms bend over the 

 margin and then gently curve and terminate on the truncated 

 side. Externally they consist of a double series of short alter- 

 nating and interlocking plates, which cover large, deep and an- 

 gular ambulacral furrows. The form of the plates in the bottom 

 of the ambulacral furrows is not determined. The central orifice, 

 from which the ambulacral furrows radiate, is sub-elliptical in 

 outline, being lengthened with the central arm elevation and cov- 

 ered externally with the arm plates. The large area above de- 

 scribed is the azygons area and the anal or ovarian pyramid and 

 orifice is situated in it, eccentrically, near the margin 



This species is founded upon four specimens, in two of which 

 the external arm plates are removed, and one of these preserves 



