1920] 



Kew: Cretaceous and Cemzoic Echinoidea 



79 



Test more or less depressed ; thickly subdiseoidal to broadly sub- 

 conical in shape ; marginal outline subcircular to subelliptical, with 

 the anteroposterior diameter usually the greater ; margin (juite thin 

 to very thick and swollen, and with or without notches in tlie ambu- 

 lacral and interambulacral areas. Petaliferous areas of the ambulacra 

 more or less elevated. Interambulacral areas more or less depressed ; 

 auxiliary groove may be present in the ambulacral areas along the 

 sides of the petals. Petals straight, extending close to the margin, 

 and wide open at their extremities. Poriferous zones nearly parallel, 

 or continuoiisly divergent, or divergent, convergent, and again diver- 

 gent ; ambulacral plates begin to broaden before end of the petal is 

 reached ; sporadic pores usually continue to the margin beyond the 

 petal proper. Apical s.ystem central or subcentral, more or less de- 

 pressed ; four genital pores present, the odd posterior one being absent. 

 Summit of test central or anteriorly eccentric. Inferior surface flat 

 or concave. Peristome central or subcentral. Periproct round, infra- 

 marginal to marginal ; never supramarginal. Main ambulacral fur- 

 rows are usually well marked, broad, and continue from the peristome 

 to the margin, often extending as faint grooves on the upper surface 

 nearly to the apical system and forming a median line on the petals. 

 Indistinct lines or grooves may be thrown off from the main furrows 

 when about two-thirds the distance to the margin, and these usually 

 extend on to the superior surface as sutural lines between the inter- 

 ambulacral and ambulacral areas. Tubereulation prominent, scrobic- 

 ular, of uniform size, or larger on the petaliferous portions of the 

 ambulacra and inferior surface. Internal structure similar to that 

 of Scutella; consists of strong radial partitions which connect with 

 the roof for about one-fourth the distance to the peristome and then 

 continue as ridges on the floor ; concentrically placed ridges are placed 

 near the margin, connecting the radial partitions ; remainder of the 

 floor more or less roughened. 



Discussion early forms of Astrodapsis, such as A. hrcxver- 

 ianus (Conrad), are closely related to Scutella. Although this species 

 shows several features characteristic of Scntella, nevertheless it is 

 placed in the genus Asirodupsis for the reason that it shows a perfect 

 gradation into true astrodapsid forms, A. hrewerianus var. diaMoensis 

 and A. cicrhocnsis. Astrodapsis is usually distinguished by having 

 either raised petals or depressed interambvdacral areas, or both. How- 

 ever, in A. brcwoiaiius both of these characters are lacking. On the 

 other hand, it has the rather thickened test, simple ambulacral furrows 



