192 MBSOZOIC and' cenozoic echinodermata of the united states. 



Dendraster arnoldi Twitchel!, n. sp. 



Plate LXXXVIII, figures 4a-d. 



Astrodapsis? sp. a Arnold, 1909, U. S. Geol. Survey Bull. 396, p. 162, PI. XXVIII, figs. 3, 3a. 

 Astrodapsis? sp. a Arnold, 1910, IT. S. Geol. Survey Bull. 398, p. 338, PL L, figs. 3, 3a. 



Determinative characters. — Test small to medium in size; subcircular to broadly oval in 

 marginal outline, usually longitudinally elongate, more rarely transversely elongate, broadest 

 centrally; upper surface greatly depressed, slightly convex, rising gradually from the mod- 

 erately thick margin to the low, central apex; under surface concave. Tubercles crowded 

 and conspicuous, notably so within the interporiferous areas and between the ambulacral 

 furrows. Apical system excentric jDosteriorly, from about one-fifth to about one-fourth 

 the radius from the center. Ambulacral petals of medium size, extending from one-half to 

 two-thirds the way to the margin, wide open at the ends, moderately wide; interporiferous 

 areas notably tumid; the posterior paired petals decidedly shorter than the rest and relatively 

 shorter than in closely related forms. Peristome small, central or subcentral; ambulacral 

 furrows straight and simple for a short distance from the peristome, then forking, the branches 

 diverging and extending nearly to the margin. Periproct very small, circular, inframarginal, 

 nearly marginal. 



Dimensions. — Type specimen: Length 45 millimeters; width 43 millimeters; height 7 

 millimeters. Small, transversely elongate specimen: Length 22.5 millimeters; width 24 

 millimeters; height 5 millimeters. 



Description. — Tins sj^ecies is found in association with D. perrini and D. coalingaensis 

 in the Etchegoin formation of California. It is rather an abundant form. A number of specimens 

 were collected by Dr. Ralph Arnold, of the United States Geological Survey, and one was 

 figured by him. The species is therefore named in his honor. The writer found several specimens 

 among the material studied which showed bifurcating ambulacral furrows, thus proving the form 

 to be allied to Scutella, instead of belonging to Astrodapsis, the genus to which Arnold doubtfully 

 referred the form he figured. The test of this species is small to medium in size, ranging from 

 less than one-half to nearly 2 inches in diameter. In marginal outline it is subcircular to broadly 

 oval, usually longitudinally elongate more rarely transversely elongate, broadest opposite the 

 middle or slightly to the rear of the middle. The upper surface is greatly depressed, slightly 

 convex, rising gradually from the margin to the apex, which is low and central or subcentral. 

 The margin is of moderate thickness, though somewhat variable, sometimes approaching the 

 thickness of that of D. perrini. The under surface is concave, the concavity gradually increasing 

 from the margin to the peristome. 



The ambulacral areas are wide, widest at the margin, where they are wider than the inter- 

 ambulacral areas; dorsal portions petaloid. The petals are of medium size, moderately wide 

 and tending to widen gradually to near their distal ends, extending from one-half to two-thirds 

 the way to the margin, the anterior pair decidedly longer than the posterior pair, the odd petal 

 slightly longer than the anterior pair; more subspatulate than subelliptical in outline; wide open 

 at the ends. The poriferous zones are wide, pores oval, pairs of pores conjugated. The inter- 

 poriferous areas are rather wide, usually from two to three times the width of the poriferous 

 zones, and notably tumid. 



The interambulacral areas are rather narrow between the petals, wide at the margin. The 

 whole surface of the test is covered with tubercles which are crowded and relatively conspicuous, 

 projecting above the edges of the rather wide scrobicules and being especially conspicuous within 

 the interporiferous areas and between the >ambulacral furrows. 



The apical system is excentric posteriorly, situated from about one-fifth to about one- 

 fourth the distance from the center to the posterior margin. The madreporite is large and there 

 are four genital pores, the anterior two being nearer together than the posterior two, and five 

 small radial plates each perforated by a minute pore. 



The peristome is small, subcircular, central, or subcentral; ambulacral furrows straight and 

 simple for a short distance from the peristome, then forking, the branches diverging and extend- 

 ing nearly to the margin. 



