88 MESOZOIC ECHINODERMATA OF THE UNITED STATES. [bull. 97. 



Hemiaster Humphreysanus Meek and Haydeu. •» 

 Plate XL VIII, Figs. la-f. 



Hemiastef ( f ) Humphreysanus Meek and Hayden, 1857. Philadelphia Acad. Nat. 



Sci. Proc, vol. It, pp. 147, 148. 

 Hemiaster (?) Humphreysanus Gabb, 1859. Cat. Invert. Fossils Cretaceous, ji. 19. 

 Hemiaster (?) Humphreysanus Meek, 1864. Smith. Misc. Coll., vol. 7 (177), p. 3. 

 Hemiaster Humphreysanus Meek. IT. S. Geol. Surv. of the Territories, vol. 9, pp. 5, 



6, PI. 10, Figs. la-(j. 

 Hemiaster Humphreysanus Clark, 1891. Johns Hopkins University Circulars, No. 



87, p. 77. 



Determinative characters. — Test oval, cordate; upper surface liigii, 

 flat, with broad, vertical truucatiou ou posterior margin, and loug 

 anterior groove; base flat; sides rounded. Ambulacra in deep furrows 

 that reach to the edge of the elevated sides. Apical disk large and 

 compact, posteriorly situated. Mouth opening small, transversely oval, 

 and bilabiate. Anal opening oval, situated high on posterior margin. 



Dimensions. — Length, 1^ inches; width, 1^ inches; height, finch. 



Description. — The absence of distinct fascioles on the type specimens 

 of this species caused Meek and Hayden to place it doubtfullj?^ in the 

 genus Hemiaster, to which beyond doubt it belongs. The test is full, tlie 

 sides rounded, and the high upper surface distinctly flattened. The 

 base is also flat, with the exception of the peristomial depression. The 

 situation of the apex so far posterior to the center of the upper surface 

 produces a long anterior sulcus, that grooves the anterior margin. A 

 high, vertically truncated surface terminates the posterior margin. 



The ambulacral areas are broad, very unequal, and deeply depressed 

 on the upper surface. The petals of the posterolateral pair are very 

 short, those of the anterolateral very long, the latter quite three 

 times the length of the former. The unpaired ambulacrum, occupies 

 the long anterior sulcus. The anterolateral pair are bent backward 

 in the upper part and forward in the lower. The i»oriferous zones are 

 broad, the pores distinct and oval (PI. xlviit, Figs, le, 1/). Since much 

 of the surface of the test has suffered removal the character of the 

 tubercles and granules can not be fully determined. 



The apical disk, which has been largely destroyed, was large, com- 

 pact, and situated far posterior to the center of the upper surface. 



The mouth opening is small and situated near the anterior margin. 

 The anal opening is oval and situated on the vertically truncated sur- 

 face of the posterior margin (PI. xlviii, Fig. Id). 



Related forms. — Hemiaster Humpkreysanus is a unique form, that is 

 not closely related to any species of Hemiaster hitherto described. 



Locality and geological horizon. — This species is from the Fort Pierre 

 formation (upper Cretaceous) of Meek and Hayden and was found 150 

 miles from the mouth of the Yellowstone river in Montana. 



Collection. — U. S. National Museum. 



