MIOCENE ECHINODERMATA. 203 



The ambulacral petals are relatively wide in proportion to their length, subelliptical, 

 nearly reaching the margin, wide open at the ends, inner two-thirds or more tumid and promi- 

 nent, outer ends nearly flush with the adjacent surfaces; poriferous zones narrow, about one- 

 third the width of the interporiferous areas; the inner row of pores elevated, pores round, 

 outer row depressed, pores slitlikc, pairs of pores conjugated. 



The interambulacral areas are deeply depressed along the middle from apex to margin. 

 The surface of the test, including the margin and interporiferous areas is covered with small, 

 conspicuous tubercles in deep scrobicules. 



The apical system is subcentral, slightly but distinctly depressed below the tops of the 

 petals. The details could not be made out on the specimen. 



The peristome is small, circular, subcentral. The main ambulacral grooves are simple, 

 straight, and deep from peristome to margin, continuing as faint lines over the margin and 

 along the ambulacral ridges nearly to the apex. The two faint processes usually given off 

 from the main grooves on species of this genus could not be clearly made out on the specimen 

 studied. 



The periproct is very small, circular, inframarginal, almost marginal. 



Related forms.- — A. tumidus is most closely related to A. wMtneyi, but has a thicker margin, 

 especially opposite the ends of the petals, is, in general, smaller, lower, and not subcorneal, has 

 only famt marginal notches, petals which are broader in proportion to then length, a more 

 depressed apical system and a less concave lower surface. A. tumidus presents some features 

 similar to A. antiselli, which make descriptions sound alike; but the forms are quite distinct, 

 A. tumidus being readily distinguished by its smaller size, less thickness, much narrower and 

 higher ambulacral ridges, and wider interambulacral depressions. 



Localities. — Kirkers Pass and 2 miles west of Walnut Creek House (Remond); San Pablo 

 Bay, Cal. (Merriam). 



Geologic horizon. — San Pablo formation (middle part), upper Miocene. 



Collection. — University of California (19424). 



Astrod apsis jacalitosensis Arnold. 



Plate XCV, figure 4. 



Astrodapsis jacalitosensis Arnold, 1909, TJ. S. Geol. Survey Bull. 396, pp. 63-64, PI. XV, fig. 5. 

 Astrodapsis jacalitosensis Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 703. 



Determinative characters. — Test large; broadly subovate in marginal outline, broadest 

 posteriorly; margin thin, deeply notched opposite the posterior petals. The whole form con- 

 siderably depressed, greatly and equally so for half the way in from edge ; centrally the upper 

 surface is higher, with conspicuous ambulacral ridges, of which the odd anterior one is the 

 highest, thus forming the anteriorly excentric apex of the test, and pronounced interambu- 

 lacral depressions which merge with the deep depression in the apical region, the posterior 

 depression being deeper and narrower than the rest. Petals tumid and elevated proximally, 

 depressed distally; inner rows of pores diverging in nearly straight lines to near the margin, 

 where wide apart outer row curving at first away from and then toward the inner row. Apical 

 system excentric anteriorly, deeply depressed. Details of lower surface could not be made 

 out. Periproct small, inframarginal, almost marginal. 



Dimensions. — Length 77 millimeters; width 74 millimeters; height 15 millimeters. 



Description. — This new Astrodapsis, which Arnold regards as a characteristic fossil of the 

 Jacalitos formation in California, is large in size, being one of the largest representatives of the 

 genus. In marginal outline it is broadly subovate, broadest posteriorly, slightly longer than 

 wide, with deep notches opposite the posterior petals and faint notches opposite the other 

 petals; margin thin. The whole form is considerably depressed, the marginal and submargi- 

 nal areas greatly so and about evenly so all around. Within the inner portion of the petaloidal 

 region there are alternating ambulacral ridges and interambulacral depressions. The depres- 

 sions extend to and merge with the deep depression in the apical region, the posterior depression 



