-CLARK.] HEMIASTEK DALLI. 89 



tp Hemiaster Dalli Clark. 



Plate XLViii, Figs. 2a-e. 



Hemiasier DalU Clark, 1891. Jolrna Hopkins University Circulars, No. 87, pp. 77. 



Determinative characters. — ^Test cordate, subhexagoiial, depressed; 

 upper surface with sharp ridges betweeu the auibulacral furrows; base 

 Hat; sides rapidly declining; anterior margin deeply grooved. Andm- 

 lacra unequal, very deeply depressed on petaloidal portions; antero- 

 lateral i)air much bent in upper part. Mouth opening transversely 

 oval, near anterior margin. Anal opening oval, situated on vertically 

 truncated surface of jjosterior margin. 



Dimensions. — Length, l^ inches; width, 1§ inches; height, § inch. 



Description. — This species is much depressed, with distinct marginal 

 angles that give it a subhexagonal outline. Prominent ridges cross 

 the upper surface betweeu the ambulacra! furroAvs. 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 ambulacral areas are very deeply dej)ressed on the upper sur- 

 face, a feature much more pronounced than in any other American 

 si»ecies. The antero-lateral pair are bent backward in their upper 

 part, and have long, deep furrows. The postero-lateral pair are about 

 one-half the length of the anterior pair. The single ambulacrum is 

 situated in the broad, deep, anterior sulcus. The j^oriferous zones in 

 the i)aiied ambulacrum are broad, the pores oval, those of each pair 

 united by a shallow furrow (PI. xlviii, Fig. 2e). In the unpaired am- 

 bulacrum the poriferous zones are narrow, the pores small and approxi- 

 mated. 



The surface is covered with small tubercles with sunken areolas 

 that increase in size toward the base, where they are large, with dis- 

 tinctly perforated mamelons and crenulated bosses. A fine microscopic 

 granulation fills the interspaces. 



The apical disk is sunken, small, and posterior to the center of the 

 upi)er surface. 



The mouth opening is transversely oval and close to the anterior 

 margin. 



The anal opening is oval and situated on the truncated surface of the 

 posterior border (PI. xlviit, Fig. 2d). 



Related forms. — The very deep ambulacral furrows and angular out- 

 line I'eadily separate i/emjos^er Dalli from any other American species. 

 It is not closely allied to any European form. 



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

 formation of the Comanche series (lower Cretaceous) of Bexar county, 

 Texas. 



Collection. — U. S. National Museum. 



