The genus astrodapsis, to which I have referred the above species, was indicated by Mr. Conrad, in the Pacific Railroad Reports, Vol. VII, Palæontological Report, page 196, without any generic description.
The genus is closely allied to clypeaster, but differs from it in having the petals of the ambulacral star extending to the edge of the disk, opened at the extremity, and in the margin of the disk being slightly notched at the extremity of each petal. The inter-ambulacral spaces are depressed. On the under surface there is a deep groove corresponding to each petal.
My collection, and that of the State Geological Survey.
A. tumidus Rémond.
Disk rounded sub-pentagonal, thick, rounded on the edge, very slightly notched at the end of the ambulacral areas.
Apex small, central, depressed.
Ambulacral petals symmetrical, very prominent, reaching to the margin, longitudinally grooved by a median line. The outer pores of the petals abruptly depressed on each side.
Inferior surface slightly concave.
Mouth central, small, rounded.
Ambulacral furrows straight, well marked.