MIOCENE ECHINODEEMATA. 191 



allied forms and less divergent than the anterior pair. Peristome small, slightly excentrio 

 posteriorly; ambtdacral 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. — Largest specimen studied: Length 58 millimeters; width 58 millimeters; 

 height 10 millimeters. Specimen A: Length 46 millimeters; width 43 millimeters; height 

 9 millimeters. Specimen B: Length 45 millimeters;, width 43 millimeters; height 9 millimeters. 



Description. — The test of this recently described California!! species is medium in size, 

 ranging from about 1^ to nearly 2 J inches in diameter. In marginal outline it is usually circular, 

 though sometimes longitudinally oval; broadest opposite the middle. The upper surface is 

 greatly depressed, slightly convex, rising gradually from the margin to the apex, which is low, and 

 slightly excentric anteriorly. The margin is unusually thick for a scutellid form and the sub- 

 marginal area beyond the ends of the petals is very narrow. The under surface is concave; the 

 concavity gradually increasing from the margin to the peristome, where it is from a fourth to 

 a third of the height of the test. 



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

 ambulacral areas; dorsal portions petaloid. The petals are large, the total area covered by 

 them being greater relatively than in most related forms, wide, and widening rapidly near the 

 apical system; broadly subelliptical in outline; the anterior pair longer than the posterior pair 

 though less so than in allied forms, the odd petal slightly longer than the anterior pair, all 

 of them extending nearly to the margin and wide open at the ends; the posterior pair less 

 divergent than the anterior pair. The poriferous zones are of moderate width, pores oval; 

 pairs of pores conjugated. The interporiferous areas are unusually wide, from three to four 

 times as wide as the poriferous zones and sometimes faintly tumid on unweathered specimens. 



The interambulacral areas are rather narrow and nearly equal in width between the petals, 

 wide at the margin. The whole surface of the test, including the margin and the interporiferous 

 areas, is covered with perforate tubercles which are relatively larger and more conspicuous than 

 in most related forms and which project above the edges of the wide scrobicules in which they 

 are placed. The tubercles are only slightly larger on the under than on the upper surface and 

 they are not very close together. 



The apical system is excentric posteriorly, though less so than in most related forms, being 

 situated from about one-sixth to about one-fifth the distance from the center to the posterior 

 margin. There is a large subpentagonal madreporite, four genital pores situated at the points 

 of the pentagon, 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, circular, slightly excentric posteriorly just beneath the apical 

 system; ambulacral furrows straight and simple for a short distance from the peristome, then 

 forking, the branches diverging and extending almost to the margin. 



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



Related forms. — D. perrini is closely related to D. arnoldi, but can be separated from it by 

 its larger petaloidal area, its wider and less tumid interporiferous areas, its broader and less 

 gradually widening petals, and its relatively longer posterior petals. 



In its circular form and its thick margin D. perrini presents an interesting resemblance to 

 MortoneUa quinquefaria, but is easily separated by its posteriorly excentric apical system, its 

 relatively larger petalloidal area .and narrower submarginal area, and by the inframarginal 

 position of its periproct. 



Lambert's genus Merriamaster is not recognized, as it is based on the erroneous idea that the 

 ambulacral furrows are simple as in Laganum. 



Locality. — Zapato Creek, one-half mile south of A. Kreyenhagen's place, Coalinga district, 

 California. 



Geologic horizon. — Etchegoin formation (apparently limited to the upper portion), upper 

 Miocene and lower Pliocene ( ?) . 



Collections. — U. S. National Museum (165560, A, B) ; Lniversity of California. 



