196 MESOZOIC AND CENOZOIC ECHINODEEMATA OP THE UNITED STATES. 



Dendeaster coalingaensis Twitchell, n. sp, 



Plate XC, figures 2a-e. 



Echinarachnius gibbsii Arnold, 1909, pars, U. S. Geol. Survey Bull. 396, pp. 34 pars, 42 pars, 162; PI. XXVIII, figs. 



4, 4a. 

 Echinarachnius gibbsii Arnold, 1910, pars, U. S. Geol. Survey Bull. 398, p. 338, PI. V, figs. 4, 4a. 



Determinative characters. — Test small; broadly ovate in marginal outline; broadest about 

 opposite or to the rear of the apical system; upper surface convex, greatly depressed though 

 less so posteriorly than anteriorly, arising directly from the thin margin to the apex which is 

 excentric posteriorly and about coincident with the apical system; under surface concave 

 around the peristome. Apical system excentric posteriorly, about one-fourth the radius from 

 the center. Ambulacral petals wide, straight and symmetrically subellipical; the posterior 

 pair shorter than the others, and usually less divergent than the anterior pair. Peristome 

 small, slightly excentric posteriorly; ambulacral furrows simple and straight for about one- 

 third the radius, then forking, each pair of branches diverging continuously nearly to the margin, 

 where they split into several small branches. Periproct small, circular, inframarginal, almost 

 marginal. 



Dimensions.- — Length 33 millimeters; width 31 millimeters; height 5 millimeters. One of 

 the smallest specimens studied was only 14 millimeters in diameter. 



Description. — This small Dendraster is found in association with D. perrini in the Coalinga 

 district, California, whence the name given. A number of specimens were collected and one 

 was figured by Arnold. The test is small, ranging from one-half to 1£ inches in diameter; 

 broadly ovate in marginal outline, broadest opposite or slightly to the rear of the apical system. 

 The upper surface is somewhat irregularly convex, greatly depressed, though less so posteriorly 

 than anteriorly, rising directly from the margin to the low, posteriorly excentric apex. The 

 margin is thin. 



The ambulacral areas are wide, widest at the margin, where they exceed the interambu- 

 lacral areas in width; dorsal portions petaloid. The petals are of medium length, wide, straight, 

 and symmetrically subelliptical, wide open at the ends; the anterior three longer than the pos- 

 terior two, the odd petal slightly longer than the anterior pair; the posterior pair usually less 

 divergent, sometimes about as equally divergent as the anterior pair. The poriferous zones 

 are wide, outer row of pores subelliptical, inner row subcircular; pairs of pores conjugated. The 

 interporiferous areas are wide, and slightly tumid. 



The interambulacral areas are narrow between the inner ends of the petals, wide from the 

 outer ends of the petals to the margin, each area of about equal width with the others. The 

 surface of the test is covered with small, uniform, perforate tubercles in deep scrobicules. The 

 tubercles and scrobicules are slightly larger on the under surface and larger than in related 

 forms. 



The apical system is excentric posteriorly, being situated about one-fourth the distance 

 from the center to the posterior border and coincident or very nearly coincident with the apex. 

 There is a large pentagonal madreporite, four genital pores of which the anterior pair are nearer 

 together than the posterior pair; and five small radial relates each perforated by a minute pore. 



The peristome is small, subcircular, slightly excentric posteriorly, but slightly less so than 

 the apical system. The ambulacral furrows are simple and straight for about one-third the 

 way to the margin, then fork symmetrically, each pair of branches diverging continuously to 

 near the margin where they split into several small branches. 



The periproct is small, circular, inframarginal, almost marginal, usually less than a milli- 

 meter from the posterior edge. 



Related forms. — D. coalingaensis is closely related to D. excentricus, of which it may be an 

 ancestral form. It differs from the latter, however, in being much smaller and less tumid 

 centrally, in having its apex less central and more nearly coincident with the apical system, 

 and in having its periproct nearer to the margin. D. coalingaensis also closely resembles D. 

 gibisii, especially the young specimens of the latter species, but can be readily separated by its 



