CLAKK.] HOLASTEK SIMPLEX. 77 



J M anler aijnplex Ga,hh, 185!i. Cat. luvort. Fossils, Cretaceous, ]>. 19. 



JJohiHfer coiuanchvii Gabb, 1851). Cat. luvort. Fossils, Crotacooiis, p. 19. 



llolastcr simplex Moek, 18G1. Smith. Misc. Coll., vol. 7, (177), p. 3. 



Uolaster comanchcsi Mook, 18G4. Smith. Misc. Coll;, vol. 7 (177), p. 3. 



Ilolaafer simplex Clark, 1891. Johns Hopkins Uuivorsity Circulars, No. 87, p. 77. 



Determinative characters. — Test oblouj^-, cordiform; upper surface 

 nuich elevated, rounded, and grooved on the anterior face; lower sur- 

 face Hat; apex nearly central. Ambulacra wide; pores unigeniinal, 

 with slit-like oi)eniugs in upper ijortion of the column that become in- 

 distinct toward the margin and upon the base. Apical disk compact 

 and not greatly elongated. Mouth opening transversely oval, situated 

 near the anterior margin. Anal oj)euiug large, on truncated posterior 

 margin. 



Dimensions. — Length, 2f inches; width, 2^ inches; height, 2 inches. 



Description. — After the most careful examination of a large number 

 of specimens the writer has been uirable to find any constant characters 

 that would warrant the separation of the forms described as Uolaster 

 simplex and Holaster comanchesi. They are accordingly included under 

 the same species. 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 an- 

 terior 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 ambulacral areas are wide. The poriferous zones of the paired 

 ambulacra are in the case of the posterior pair bent more or less for- 

 ward in their upper portions, and in the case of the anterior slightly 

 backward. The pores of the paired ambulacra have elongated o[»en- 

 ings in the upper portion (PI. xxxviii. Fig. Id) and become very small 

 and indistinct below (PI. xxxviii. Fig. le). The pores of the unpaired 

 ambulacrum are exceedingly small. 



The surface of both interambulacral and ambulacral plates are cov- 

 ered by small tubercles, between which are numerous microscopic 

 granules (PI. xxxix, Figs. Ic, Id). 



The apical disk is compact and rather short. The four genital plates 

 are large and broad. The two interposed oculars are large; the re- 

 maining three very small (PI. xxxviii. Fig. 1/). 



The mouth opening is large, transversely oblong, and situated near 

 the anterior margin (PI. xxxviii. Fig. lb). The anal opening is oval, 

 acuminate above, and situated on the truncated posterior border (PI. 

 XXXIX, Fig. 1^). 



Related forms. — Holaster simplex is separated from Gardiaster ductus 

 by its more elevated upper surface and shallow anterior sulcus. It is 

 yavy like Holaster Iwvis, vnr. planus, from the European Cretaceous, but 

 is more elevated and distinctly broader posteriorly. 



Locality and geological horizon. — This species is from the Washita 

 formation of the Comanche series (lower Cretaceous) of Texas, It is 



