CI ARK] HEMI ASTER TEX ANUS. 87 



Determinitive characters. — Test oval, cordate, declining- anteiioily, 

 elevated slightly posteriorly; anterior sulcus broad and deej), produc- 

 ing groove in margin. Ambulacra broadly deiircssed on upper surface; 

 anterolateral pair bent backward in up[>('r part; unpaired ambula- 

 crum very broad. Apical disk compact, tUe four genitals distinctly 

 perforated. Mouth t>pening large, transversely oval, bilabiate. Anal 

 opening large, oval, at center of truncated surface of posterior margin. 



JJimensionti. — Length, 1^ to 2 inches; width, 1^ to If inches; height, 

 § to 1 inch. 



Description. — This im])ortant si)ecies is broadly cordate and slightly 

 depressed on the ujiper surface; base Hat. Posterior to the apex there 

 is a sharp ridge that declines gradually toward the obliquely truncated 

 margin. Anteriorly the upper surface declines toward the anterior 

 border. 



The ambulacral areas are broad, very unequal, and depressed in the 

 petaloidal portions. The poriferous iones are broad, the pores of the 

 paired ambulacra elongated (PI. xlvii. Fig. le), those of the single am- 

 bulacrum round and approximated (PI. XLvri, Fig. 1/). The petals of 

 the anterolateral pair are bent backward in their ui)per part and are 

 nearly twice the width of the i)Ostero -lateral j)air. The single ambu- 

 lacrum is very broad, the poriferous zones widely separated. 



The surface of the test is covered with small tubercles that are per- 

 forated and for the most part crenulated (PI. xlvii, Fig. lA, It). A 

 microscoi)ic granulation tills the intertubercular S])ace. 



The apical disk is small and compact and situated near the center. 

 The four genital plates are distinctly perforated ; the anterolateral is 

 larger than the others and serves as the madreporite. The live oculars 

 are wedged between the genitals and are deeply cut by the upper por- 

 tions of the ambulacra (PI. xlvii. Fig. Ig). 



The mouth opening is large, transversely oval, and bilabiate. The 

 anal opening is large, oval, and situated in the center of the truncated 

 surface of the posterior margin (PI. xlvii, Fig. 1^/). 



Related forms. — Under the name of Hemiaster americanus, Giebel pub- 

 lished the description of a new species of Hemiaster, in 1853, that he 

 states can be with difficulty distinguished froin R. texanus. It is con- 

 sidered identical in the present report. The differences seem hardly to 

 warrant the establishment of a new si)ecies. Periasfer anstraUs, dei^- 

 cribed by Gabb, from Peru, so far as the figures and description show, 

 must be a Hemiaster, and to all appearances is closely allied if not 

 identical with H. texanus. 



Locality and geological horizon. — Hemiaster texanus is from the upper 

 division of the Cretaceous of Texas. It is characteristic of the Austin 

 chalk. 



Collections.— \J. S. National Museum; Prof. Kobt. T. Hill. 



