128 



University of California Publications in Geology [Vol. 12 



DENDRASTER PACIFICUS Kew, n. sp. 

 Plate 33, figures la, lb, Ic 



Cotypes.— No. 448 Calif. Acad. Sci. Coll. Pal. and no. 11340 Univ. 

 Calif. Coll. Invert Pal. 



Test small. Average measurements of liolotype : anteroposterior 

 diameter 36 mm., transverse diameter 35 mm., greatest height 8.3 mm. 

 Outline suhpentagonal. Upper surface depressed, and rising gently 

 from a moderately thickened and rounded margin to a low apex 

 situated slightly anterior to the center of the test. Very indistinct 

 depressions are present in the interambulacral areas. Ajiical system 

 slightly eccentric to the posterior. Petals of the trivium of the same 

 length and longer than those of the bivium, extending a little over 

 half the distance from the apical system to the margin. Rows of 

 pores diverge for about one-half the length of the petal and then 

 continue in parallel lines to the end, the outer rows converging but 

 little in the distal portion of the petal ; a few sporadic pores extend 

 beyond nearly to the edge of the test. Odd anterior petal is slightly 

 narrower than the lateral petals. Petals are more or less elevated, this 

 being a distinctive character. Four large genital pores and five per- 

 forated radial plates are present in the apical system. Lower surface 

 concave to the peristome ; the latter is subcircular in outline and 

 posterior to the center of the test, being opposite the apical system. 

 Distinct, broad ambulacral furrows present, branching dichotomously 

 a short distance from the mouth and continuing to the margin. Tu- 

 berculation prominent ; consists of a few large scrobicular tubercles 

 on the petals; smaller secondary tubercles are present over the re- 

 mainder of the upper surface, interspaced with numerous milliaries ; 

 the under side is covered by a few large tubercles of the same size as 

 on the petals, and interspaced with numerous smaller ones. Periproct 

 round, inframarginal, and situated a distance about equal to its own 

 diameter from the edge of the test. 



Related forms. — This form seems to be closely related to Dendraster 

 arnoldi Twitchell, but differs in that it has a less eccentric apical 

 system ; has shallow interambulacral grooves on the upper surface ; 

 and possesses a distinctly pentagonal outline. It may be distinguished 

 from other west coast dendrasters on account of its large tubercles, 

 very slightly eccentric apical system, and a swollen margin. From D. 



