96 MESOZOIC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



also flat, with the exception of the peristomial depression. The situation of the apex so far pos- 

 terior 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 ambulacra 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 poriferous zones are broad, the pores distinct and oval. 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 system, which in the specimen examined has been largely destroyed, was evi- 

 dently large, compact, and situated far posterior to the center of the upper surface. 



The peristome is small and situated near the anterior margin. The periproct is oval and 

 situated on the vertically truncated surface of the posterior margin. 



Related forms. — H. humphreysanus is a unique form and is not related to. any species of 

 Hemiaster hitherto described. 



Locality. — One hundred and fifty miles from the mouth of the Yellowstone River in Mon- 

 tana (Meek and Hayden). 



Geologic horizon. — Pierre shale, of Montana group, Upper Cretaceous. 



Collection. — U. S. National Museum (331). 



Hemiaster californicus Clark. 



Plate L, figures la-d. 



Hemiaster californicus Clark, 1891, Johns Hopkins Univ. Circ, vol. 10, No. 87, p. 77. 

 Hemiaster californicus Clark, 1893, Johns Hopkins Univ. Circ, vol. 12, No. 103, p. 52. 

 Hemiaster calif 'ornicus Clark, 1893, U. S. Geol. Survey Bull. 97, p. 90, PL XLIX, figs. la-c. 



Determinative characters. — Test small, subquadrate, depressed; upper surface flat, with 

 shallow ^anterior sulcus, sloping sides, and low truncated posterior margin; lower surface flat. 

 Ambulacra depressed, in shallow furrows. Apical system small, posterior to center. Peristome 

 transversely oval near anterior margin. Periproct situated on the low truncated surface of the 

 posterior margin. 



Dimensions. — Length 28 millimeters; width 27 millimeters; height 11 millimeters. 



Description. — This form has the details of its structure poorly preserved in the specimens 

 examined, although it belongs, beyond much doubt, to the genus Hemiaster. The test is small, 

 much depressed, and subquadrate in ambital outline. It is broader anteriorly, with a feeble 

 groove in the center of the anterior margin. The posterior portion of the test is but slightly 

 more elevated than the anterior, and the truncated margin is low and narrow. The base is flat, 

 with the exception of the peristomial depression. 



The ambulacra are but slightly depressed in the petaloidal portions. The poriferous zones 

 are broad, and the oval pores are united by shallow furrows. The apical system is small and 

 situated slightly forward of the center. The peristome is transversely oval and situated near 

 the anterior margin. The periproct is small, oval, and situated on the low, truncated surface of 

 the posterior border. 



Related forms. — From its shallow ambulacra! furrows, Hemiaster californicus may be com- 

 pared to H. humphreysanus, but is readily separated from it because of its more central apical 

 system and its depressed form. 



Locality. — Redding, Shasta County, Cal. 



Geologic horizon.— Chico formation, Upper Cretaceous. 



Collection. — U. S. National Museum (31203). 



Hemiaster beecheri Clark, n. sp. 



Plate L, figures 2a-c. 



Determinative characters. — Test large, cordiform, elevated; upper surface inflated, depressed 

 posteriorly; ambulacra broadly depressed on dorsal surface, anterior ambulacrum in broad 

 sulcus. 



