198 MESOZOIC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



Astrodapsis antiselli Conrad. 



Plate XCIV, figures 3, 4a-b. 



Astrodapsis antiselli Conrad, 1856, Acad. Nat. Sci. Philadelphia Proa, vol. 8, p. 315; no figures. 



Astrodapsis antiselli Conrad, 1857, U. S. Pacific P. R. Expl., vol. 7, p. 196, PI. X, figs. 1, 2. 



Astrodapsis antiselli Meek, 1864, Smithsonian Misc. Coll., vol. 7 (183), p. 2. 



Astrodapsis antiselli Gabb, 1S69, California Geol. Survey Bept., Paleontology, vol. 2, p. 110. 



Astrodapsis antiselli Cooper, 1888, State Mineralogist California Seventh Bept., p. 270. 



Astrodapsis antiselli Arnold, 1908, U. S. Nat. Mus. Proa, vol. 34, PI. XXXV, fig. 10; no description. 



Astrodapsis antiselli Arnold, 1909, U. S. Geol. Survey Geol. Atlas, Santa Cruz folio (No. 163), PI. II, fig. 58. 



non Astrodapsis antiselli var. arnoldi Pack, 1909, Univ. California Dept. Geology Bull., vol. 5, No. 18, pp. 279-281, PI. 



XXIV, figs. 1, 2. See Astrodapsis arnoldi. 

 Astrodapsis antiselli Stefanini, 1911, Soc. geol. italiana Boll., vol. SO, p. 702. 



Determinative characters. — Test medium in size; regularly oval in marginal outline, longer 

 than broad, slightly truncated at anterior end, slightly pointed at posterior end, with faint 

 notches opposite ends of petals; margin rounded and very thick, almost as thick as rest of 

 test. The whole form is considerably depressed, almost equally so from edge to edge, and 

 therefore subdiscoidal ; the upper surface with broad, flattened ambulacral ridges alternating 

 with narrow interambulacral depressions; apex excentric anteriorly, in front of depressed 

 apical system; lower surface slightly concave. Ambulacral petals large, broad, tumid, espe- 

 cially near apical system; poriferous zones narrow, at first diverging, then converging sligbtly 

 from one-fourth to one-third the way to the margin, and again diverging to the wide-open ends 

 which are nearly at the margin. Peristome central; the main ambulacral grooves straight, 

 well denned, and rather deep from peristome to margin and continuing as faint lines over 

 margin to near apex, two faint lines are given off about halfway to margin, which continue 

 over margin to near apical system. Periproct small, inframarginal, almost marginal. 



Dimensions. — Specimen B: Length 57 millimeters; width 50 millimeters; height 14 

 millimeters. 



Description. — This species was one of the first of the Tertiaxy echinoids to be reported 

 from the Pacific coast of the United States, having been described by Conrad as early as 1856. 

 Conrad founded his new genus Astrodapsis upon this species. The test is medium in size, 

 averaging about 2 inches in length. In marginal outline it is regularly oval, longer than 

 broad, usually slightly truncated at the anterior end and somewhat pointed at tbe posterior 

 end, with very slight notches opposite the ends of the petals; margin rounded and very thick, 

 almost equal in thickness to the rest of the test, somewhat thicker anteriorly than poste- 

 riorly. The whole form considerably depressed, subdiscoidal; the upper surface with broad 

 flattened ambulacral ridges alternating with narrow interambulacral depressions; apex excen- 

 tric anteriorly in front of apical system; under surface slightly concave. 



The ambulacral areas are wide; dorsal portions petaloid. The petals are large, broad, 

 tumid, widest and most tumid near the apical system, constituting broad ridges, all of them 

 extending nearly to the margin, wide open at the ends ; interporif erous areas very wide ; porif- 

 erous zones narrow and of nearly equal breadth throughout their length, at first diverging, 

 then converging slightly from one-fourth to one-third the way from the margin, and again 

 diverging; inner row of pores round, outer row broadly slitlike, pairs of pores conjugated. 



The interambulacral areas are narrow, decidedly depressed, forming radial grooves in 

 the upper surface. The whole surface of the test is covered with small but conspicuous 

 tubercles, which make the surface feel rough; they are larger and more conspicuous in the 

 interporif erous areas. 



The apical system is slightly excentric anteriorly, in a star-shaped depression below the 

 level of the ambulacral ridges. There appear to be only four genital pores which are located 

 at four of the points of the star-shaped depression. No further details could be made out. 



The peristome is central, circular to subpentagonal. The main ambulacral grooves are 

 straight, well defined and rather deep from peristome to margin and continue as faint lines over 

 the margin and along the middle of the ambulacral ridges to the apical system; two faintly 



