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Vniversity of California Puhlicatiom in Geology [Vol. 12 



center of the test, the amount of eccentricity being less than one- 

 seventh the radius of the test. Ambulacra wider than the interam- 

 bulacra at the ambitus. Petals of moderate width and extending 

 about two-thirds the distance to the margin. Petals of the bivium 

 shortest, with the odd anterior one longer than the two anterior lateral 

 ones. Pores oval and conjugated, rows of pores diverging rela- 

 tively rapidly at first and then continuing to the ends of the petals, 

 converging but little at their extremities ; the anterior rows of the 

 lateral petals are curved considerably more than the posterior rows, 

 which in some specimens are nearly straight. All the petaliferous 

 areas are distinctly tumid. Poriferous zones wide, each being equal to 

 about two-thirds the width of the interporiferous area. Madreporie 

 area large ; four large genital pores present and also five radial plates, 

 each perforated by a small pore. Inferior surface concave to the peris- 

 tome. Mouth posteriorly eccentric and situated approximately oppo- 

 site the apical system. Ambulacral furrows broad, well marked, 

 branching dichotomously a short distance from the peristome and con- 

 tinuing to the margin. Periproct small, round, inframarginal, and 

 situated a distance equal to its own diameter from the edge of the test. 

 Tubercles scrobicular and i^rominent ; those of the upper surface are 

 larger on the petaliferous area, whereas those on the remainder of the 

 upper surface are smaller, but increase in size toward the marginal 

 area. Tubercles on the inferior surface deeply scrobicular and of 

 the same size as those on the petals, except in the ambulacral furrows, 

 where they are markedly smaller. The internal structure consists of 

 radial partitions extending inward about one-third the distance to 

 the center with a few broad supports near the margin arranged con- 

 centrically in the ambulacral areas between the partitions ; the re- 

 maining portion of the floor is smooth. Auricles low but heavily 

 built and connected at their base by a low, broad ridge surrounding 

 the edge of the actinostome. Jaws clypeastroid in character and 

 rather flat. 



Related forms. — Dendraster arnoldi in general appearance resem- 

 bles D. perrini (Weaver), but differs from the latter in that the 

 tubercles are relatively smaller and of the same size over the test, 

 whereas in the former they are prominent, deeply scrobicular, and vary 

 in size, those on the interporiferous areas of the petals being larger 

 than those outside ; the petals are more symmetrical and comparatively 

 broader in D. perrini; and as a whole the test is larger and thicker. 



