CRETACEOUS ECHINODEEMATA. 85 



The peristome is near the posterior margin. The periproct is not preserved. 

 Locality. — Twenty-five miles southwest of Mingusville (now Wibaux), Mont. 

 Geologic horizon. — Fox Hills ( ?) sandstone of Montana group, Upper Cretaceous. 

 Collection. — U. S. National Museum (31201). 



Genus HOLASTER Agassiz. 



Holaster simplex Shumard. 



Plate XXXIV, figures 3a-b; Plate XXXVIII, figures la-j; Plate XXXIX, figures la-g. 



Holaster simplex Shumard, 1853, Pal. Expl. Red River of Louisiana in 1852, p. 210, PL III, fig. 2. 



Holaster comanchesi Marcou, 1858, Geology of North America, p. 40, PL III, fig. 3. 



Holaster simplex Desor, 1858, Synopsis des echinides fossiies, p. 450. 



Holaster comanchesi Desor, 1858, Synopsis des echinides fossiies, p. 449. 



Holaster simplex Gabb, 1859, Cat. Invert. Fossils Cretaceous, p. 19. 



Holaster comanchesi Gabb, 1859, Cat. Invert. Fossils Cretaceous, p. 19. 



Holaster simplex Meek, 1864, Smithsonian Misc. Coll., vol. 7 (177), p. 3. 



Holaster comanchesi Meek, 1864, Smithsonian Misc. Coll., vol. 7 (177), p. 3. 



Holaster simplex Clark, 1891, Johns Hopkins Univ. Circ, vol. 10, No. 87, p. 77. 



Holaster simplex Clark, 1893, Johns Hopkins Univ. Circ, vol. 12, No. 103, p. 52. 



Holaster simplex Clark, 1893, U. S. Geol. Survey Bull. 97, pp. 76-78, Pis. XXVIII, figs, la-g; PI. XXXIX, figs. la-d. 



Holaster completus Cragin, 1893, Geol. Survey Texas Fourth Ann. Rept., pp. 155, 156, PI. XXIV, fig. 10; PL XXV, 



fig. 14; PL XXVI, figs. 6-8. 

 Holaster nanus Cragin, 1893, Geol. Survey Texas Fourth Ann. Rept., p. 156, PI. XXIV, fig. 14; PL XXVI, fig. 11. 

 Holaster simplex Cragin, 1893, Geol. Survey Texas Fourth Ann. Rept., p. 157, PL XXV, fig. 13. 

 Holaster supernus Cragin, 1893, Geol. Survey Texas Fourth Ann. Rept., pp. 157, 158. 

 Holaster completus Cragin, 1894, Colorado Coll. Studies, vol. 5, pp. 45, 47. 

 Holaster nanus Cragin, 1894, Colorado Coll. Studies, vol. 5, pp. 45, 47. 



Determinative characters. — Test oblong, cordiform; upper surface much elevated, rounded, 

 and grooved on the anterior face; lower surface flat; apex nearly central. Ambulacra wide; 

 pores unigeminal, with slitlike openings in the upper portion of the column that become indis- 

 tinct toward the margin and upon the base. Apical system compact and not greatly elongated. 

 Peristome transversely oval, situated near the anterior margin. Periproct large, on truncated 

 posterior margin. 



Dimensions. — Length 32 to 70 millimeters; width 30 to 60 miUimeters; height 14 to 53 

 millimeters. 



Description. — The test is oblong, cordiform, much elevated, rounded above and flattened 

 below. A shallow groove occupies the anterior face of the test, producing a feeble emargination 

 of the anterior ambitus. A slight ridge extends along the line of the central suture of the single 

 interambulacrum, which is suddenly terminated by the truncated face of the posterior margin. 



The ambulacra are wide. The poriferous zones of the paired ambulacra are in the case 

 of the posterior pair bent more or less forward in their upper portions, and in the case of the 

 anterior slightly backward. The pores of the paired ambulacra have elongated openings in the 

 upper portion and become very small and indistinct below. The pores of the unpaired ambula- 

 crum are exceedingly small. 



The surface of both interambulacraband ambulacral plates are covered by small tubercles, 

 between which are numerous microscopic granules. 



The apical system is compact and rather short. The four genital plates are large and 

 broad. The two interposed oculars are large, the remaining three very small. 



The peristome is large, transversely oblong, and situated near the anterior margin. The 

 periproct is oval, acuminate above, and situated on the truncated posterior border. 



The variations in this species are very pronounced, some of the individuals being high and 

 globose and others low and flat. A critical examination of a large number of specimens affords 

 no satisfactory criteria for the recognition of the new species established by Marcou and Cragin. 



Related forms. — H. simplex is very like H. Isevis var. planus, from the European Cretaceous, 

 but is more elevated and distinctly broader posteriorly. 



