Geol.-Vol. I.] MERRIAM— TERTIARY SEA-URCHINS. 167 



Specimens ranging up to 45 mm. in diameter, average between 30 and 35 

 mm. Test circular to elliptical, depressed; margins, particularly in old indi- 

 viduals, inclined to be thickened and rounded, usually notched at the ends 

 of the ambulacral areas. 



Ambulacra always more or less elevated and showing, on perfectly pre- 

 served specimens, a faint groove running from the marginal notch more than 

 half-way up to the apical system. Petals wide open at the ends, pores con- 

 tinuing almost to the margins. Inner rows of rounded pores only slightly 

 convergent near the margin. Elongated pores of outer rows converging 

 near the margin, becoming rounded and running parallel with the inner rows 

 from that point. 



Anus marginal or inframarginal. On well preserved specimens the inferior 

 surface shows well marked, straight, undivided ambulacral grooves, which 

 pass into the marginal notches and extend themselves on the upper surface, 

 forming the median groove of the ambulacral areas. The tubercles are not 

 noticeably different in size on the upper and lower surfaces. 



The internal skeleton consists of a pair of strong, radially placed plates, 

 extending half the distance from the margin to the center in each interam- 

 bulacral space. 



San Pablo formation, excepting the lowest beds. 



Astrodapsis Whitneyi Rcuiond. 

 Plate XXI, Figs. 4 and 4a. 



Astrodapsis Whitneyi Remond., Proc. Cal. Acad. Sci., Vol. Ill, 1863-67, 



P- 52. 

 Astrodapsis Whitneyi. Figured by Gabb., Geol. Surv. Cal., Palaeontology, 

 Vol. II, 1869, p. 37, PI. XIII, figs. 67 and 67a. 



The average specimens of this species are considerably larger than those 

 of A. tmnidus. The largest specimen examined measured between 60 and 

 65 mm. in diameter. Test circular, strongly arched above; margin thin; 

 marginal notches at the ends of the ambulacra deep. 



Petals considerably elevated, with median groove running from the mar- 

 ginal notch toward the apex, pores similar in form and arrangement to those 

 of A. tuinidus. 



Ambulacral furrows of the inferior surface well marked, straight, undivided. 

 Anus inframarginal. Tubercles not differing materially on the upper and 

 lower surfaces of the test, frequently smaller than in tumidus. 



San Pablo formation, probably in the upper beds. 



