MIOCENE ECHINODERMATA. 199 



defined processes are given off about halfway from the peristome to the margin, which extend 

 to and over the margin and on the upper surface between the top of the ridges and the bottom 

 of the depressions, becoming indistinct as the apical system is approached. 



The periproct is small, subcircular, inframarginal, almost marginal. 



Related forms. — Duncan 1 regarded Conrad's genus Astrodapsis as a synonym of Breynius's 

 genus Arachnoides; but, though there are certain points of resemblance, the inframarginal 

 position of the periproct, the more or less depressed apical system and the elevated, ridgelike 

 ambulacra! areas alternating with depressed groovelike interambulacral areas on the upper 

 surface would appear to sufficiently differentiate the genus Astrodapsis. 



The present species is readily separated from the other representatives of the genus, 

 although some confusion has arisen in the literature and in various collections because of the 

 obscure location of Conrad's description and figures. A. whitneyi differs in being broadly sub- 

 conical and in having a thin margin, more circular outline, more pronounced marginal notches, 

 higher ambulacral ridges and deeper interambulacral depressions. A. tumidus differs in being 

 smaller and in having a greater difference between the height of the ambulacral ridges and 

 interambulacral depressions. 



Localities. — Monterey County (type), and 2 miles south of San Lucas, Monterey County 

 (specimen B), Cal. Conrad also cites Estrella, and Gabb gives near Buena Vista Lake, both in 

 California. 



Geologic horizon. — Santa Margarita formation, middle or upper Miocene. 



Collection. — U. S. National Museum (165466, A; 165466a, B). (Both the type and the 

 specimens collected by Arnold, which include specimens A and B.) 



Astrodapsis aenoldi Twitchell, n. sp. 



Plate XC V, figure 1. 



Astrodapsis whitneyi Arnold, 1909, IT. S. Geol. Survey Bull. No. 396, p. 63, PL XI, fig. 1. 



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



XXIV, figs. 1, 2. 

 Astrodapsis whitneyi Stefanini, 1911, pars, Soc. geol. italiana Boll., vol. 30, p. 703. 



Determinative characters. — Test large, larger than most representatives of the genus; circu- 

 lar in marginal outline ; margin and wide submarginal area notably thin, undulating, with pro- 

 nounced posterior notches and less pronounced anterior notches. The whole form considerably 

 depressed, greatly and nearly equally so for half the distance from margin toward the center, . 

 moderately elevated centrally, with rather broad and somewhat flattened ambulacral ridges 

 alternating with broad deep interambulacral depressions. The inner ends of the ambulacral 

 ridges almost coalesce around the slightly depressed apical system and form the somewhat 

 flattened subcentral apex of the test. Lower surface flat or nearly so. Petals moderately 

 wide, elongate subelhptical, nearly reaching the margin where wide open; inner halves elevated 

 and tumid, outer halves depressed and merging with submarginal area; inner rows of pores 

 of each petal nearly parallel for most of their length. Peristome small, circular, central; main 

 ambulacral grooves simple, straight and deep to the margin, continuing as faint lines straight 

 and deep over margin and along middle of ambulacral ridges nearly to apex; two faint processes 

 are given off about half way to margin winch continue over margin and on upper surface to near 

 the apex. Periproct very small, inframarginal, almost marginal. 



Dimensions. — Length 76 millimeters; width 75 millimeters; height 10 millimeters. 



Description. — The test of this new Astrodapsis is unusually large. In marginal outline it 

 is circular; the margin is markedly thin, undulating, with well-defined notches opposite the 

 ends of the petals, the posterior notches being slightly larger than the anterior pah, which are 

 larger than the odd one; the submarginal area for nearly half the way toward the center is also 

 thin, about as thin as at the margin. Centrally, within the area circumscribed by the thin and 

 flattened submarginal area, the upper surface is moderately elevated with rather broad and 



1 Duncan, P.M., Eevislon of genera ana great groups of the Echinoidea: Linn. Soc. Jour. Zoology, vol. 23, p. 165, 1S91. 



