CRETACEOUS ECHINODERMATA. 91 



Description. — This species is much depressed, with distinct marginal angles that give it a 

 subhexagonal outline. Prominent ridges cross the upper surface between the ambulacra! fur- 

 rows. The anterior sulcus cuts deeply into the test and grooves the anterior margin. The sides 

 decline rapidly to the flattened base. The low posterior surface has a nearly vertical truncation 

 of small area. 



The ambulacra are very deeply depresssd on the upper surface, a feature much more pro- 

 nounced than in any other American species. The anterolateral pair are bent backward in 

 their upper part, and have long, deep furrows. The posterolateral pair are about one-half the 

 length of the anterior pair. The single ambulacrum is situated in the broad, deep, anterior 

 sulcus. The poriferous zones in the pahed ambulacra are broad, the pores oval, those of each 

 pair united by a shallow furrow. In the unpaired ambulacrum the poriferous zones arc narrow, 

 and the pores small and approximated. 



The surface is covered with small tubercles with sunken areolas that increase in size toward 

 the base, where they are large, with distinctly perforated mamelons and crenulated bosses. A 

 fine microscopic granulation fills the interspaces. 



The apical system is sunken, small, and posterior to the center of the upper surface. 



The peristome is transversely oval and close to the anterior margin. The periproct is oval 

 and situated on the truncated surface of the posterior border. 



Related forms. — The very deep ambulacral furrows and angular outline readily separate 

 H. dalli from any other American species. It is not closely allied to any European form. 



Locality.— Bexar County, Tex. 



Geologic horizon. — Washita group of the Comanche series, Cretaceous. 



Collection. — U. S. National Museum (19114). 



Hemiaster calvini Clark. 



Plate XLVII, figures 2a-i. 



Hemiaster calvini Clark, 1S93, Johns Hopkins Univ. Circ, vol. 12, No. 103, p. 52. 

 Hemiaster calvini Clark, 1S93, U. S. Geol. Survey Bull. 97, pp. 30, 91; PI. XLIX, figs. 2a-i. 



Determinative characters. — Test oval, cordate, inflated; upper surface elevated; lower sur- 

 face flat; posterior margin obliquely truncated. Ambulacra moderately depressed on the upper 

 surface. Peristome transversely oval. Periproct oval, high on truncated posterior border. 



Dimensions. — Length 21 to 38 mdlimeters;* width 20 to 35 millimeters; height 16 to 33 

 millimeters. 



Description. — This species has an elevated test that is oval, cordate, and broadly truncated 

 on the posterior margin. The anterior sulcus is shallow and produces but a feeble groove on the 

 anterior margin. The base is flat. 



The ambulacra are narrow. The poriferous zones are broad; the petals moderately 

 depressed and of good length; the pores in the petaloidal portions of the paired ambulacra 

 elongated, slitlike, those of the unpaired ambulacrum small and approximated. Beyond the 

 petals the pores are very small, and occupy the lower outside corner of each plate. 



The surface of the plates is covered with small perforated tubercles rather widely scattered 

 between which a microscopic granulation intervenes. The peripetalous fasciole is indistinct. 



The apical disk is small, compact, and nearly central. 



The peristome is small, transversely oval, and situated at a distance from the margin. The 

 periproct is oval and situated high on the truncated surface of the posterior margin. 



Related forms. — H. calvini is most closely related to H. whitei, from which, however, it is 

 distinguished by its more elevated form and its small, narrow, anterior sulcus. The apical 

 system is also more excentric. 



Localities. — Shoal Creek (type), near Denison, near El Paso, and other localities in Texas. 



Geologic horizon. — Washita group of the Comanche series, Cretaceous. 



Collections. — U. S. National Museum (12241); University of Iowa. 



