154 University of California Publications in Geology [Vol. 12 



SCHIZASTER STALDERI Weaver 

 Plate 42, figures 3a, 3b, 3c, 3d 



Schizaster (?) stalderi Weaver. Univ. Calif. Publ. Bull. Dept. Geol., vol. 



5, 1908, p. 274, pi. 21, fig. 3. 

 ScMsaster stalderi. Stefaniui, Boll. Soc. geol. ital., vol. 30, 1911, p. 705. 

 Schizaster {?) stalderi. Clark and Twitchell, U. S. Geol. Surv. Mon., vol. 



54, 1915, p. 221, pi. 103, fig. 2. 



Holotype.—No. 31019 Univ. Calif. Coll. Invert. Pal. 



Test attaining rather large size. Average measurements : antero- 

 posterior diameter 26 mm., transverse diameter 39 mm., greatest height 

 15 mm. Marginal outline subelliptical to subcordate, with the greatest 

 width anterior to the center of the test. Apical system posteriorly 

 eccentric. Summit coincides with the apical system or is immediately 

 posterior to it ; i:)osterior odd interambulacruni elevated slightly to 

 form an indistinct ridge. Ambulacra petaloid ; petals of bivium 

 slightly less than one-half the length of the anterolateral petals, and 

 extending about three-fifths the distance to the margin. Inner rows 

 of pores in paired petals converge but little at their extremities; outer 

 rows converge close to the inner rows near the end of the petals. An- 

 terior pair of lateral petals extend close to the margin and are narrower 

 than the posterior jjair. Interporiferous areas of lateral petals narrow, 

 being about the same width as one of the poriferous areas. Odd 

 anterior petal considerably wider than the others, due to the greater 

 width of the interporiferous area ; extends nearly to the anterior 

 margin; rows of pores converge but little at their extremities. All 

 petals situated in deep sulci, the anterior pair being slightly shallower 

 than the others ; the odd anterior one continues to the under side, cut- 

 ting the anterior margin in a prominent notch. Inferior surface flat 

 or somewhat convex ; episternum prominent and elevated. Peristome 

 placed close to anterior margin in a deep transverse groove which 

 extends almost across the test. Periproct large, round, and situated 

 well up on the posterior truncation of the test. Tubercles similar 

 over the test, except on the actinal plastron where they are larger and 

 less crowded. 



Belated forms. — This form does not seem to be closely allied to 

 any other Schizaster on the Pacific Coast. It is easily distinguishable 

 from Schizaster lecontei Merriam by its more depressed test, longer 

 petals in the bivium, and by the presence of an actinal transverse 

 groove in which the peristome is situated. 



