54 MESOZOIC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



Suborder DUDEMINA. 



Family ORTHOPSID^I. 



Genus ORTHOPSIS Cotteau. 



Orthopsis planulata Clark, n. sp. 



Plate XVI, figures 2a-c. 



Determinative characters. — Test small, depressed, circular in ambital outline, upper surface 

 flattened, lower surface concave. Ambulacra narrow, pore pairs simple, primary tubercles 

 small. Interambulacra broad, with several rows of vertical primary tubercles slightly larger 

 than the ambulacra. Peristome of medium size. 



Dimensions. — Diameter 28 to 32 millimeters; height 10 to 12 millimeters. 



Description. — The species is represented by two specimens embedded in a single mass of 

 indurated marl, one showing the adoral and the other the aboral surface, the former containing 

 a number of spines. The test is relatively small, much depressed, the upper surface being dis- 

 tinctly flat, except directly at the apical system, which is slightly elevated. The under surface 

 is concave. The ambulacra are narrow and bear two rows of small perforate, uncrenulated 

 tubercles that become indistinct toward the apical system. The plates are simple primaries on 

 the upper surface, but from just above the ambitus to the peristome two of the primaries com- 

 monly combine, the suture passing through the interporiferous tubercle. The poriferous zones 

 bear a straight uniserial series of pores. 



The interambulacra are broad, with six rows of perforate, uncrenulated tubercles on the 

 under surface which are much reduced on the upper surface. 



The peristome is of moderate size. 



Locality. — Cut on Bonham road, southeastern edge of Denison, Tex. 



Geologic horizon. — Grayson marl member of the Denison formation, of Washita group, 

 Comanche series, Cretaceous. 



Collection. — Johns Hopkins University '(T 3005). 



Orthopsis occidentalis Cragin. 

 Plate XVII, figures la-g. 



Orthopsis occidentalis Cragin, 1893, Geol. Survey Texas Fourth. Ann. Kept., pp. 160, 161, Pis. XXV, figs. 1-3; XXVII, 

 fig. 2. 



Determinative characters. — -Test large, depressed, slightly pentagonal in ambital outline, 

 upper surface tumid, lower surface concave; five perforated genitals, one a large madreporite, 

 some of the oculars reaching the periproctal margin. Ambulacra narrow, straight; pore pairs 

 in simple, straight series; primary tubercles small, perforate, and smooth; interambulacra broad 

 with several vertical rows of small primary tubercles similar to the ambulacra. Peristome of 

 medium size. 



Dimensions. — Diameter 57 millimeters; height 33 millimeters. 



Description. — -The test is large, somewhat depressed, and slightly pentagonal in ambital 

 outline. The upper surface is tumid directly above the ambitus, while the lower surface is con- 

 cave. 



The. ambulacra are very narrow. Two rows of small primary perforated and uncrenulated 

 tubercles extend from the peristome to the apical disk. The plates are simple primaries on the 

 upper surface, but from the ambitus to the peristome two of the primaries generally combine, the 

 suture passing through the interporiferous tubercle. The pores occur in simple series in the 

 narrow poriferous zones. 



The interambulacra are much broader than the ambulacra, with six rows of primary tubercles 

 similar to those in the ambulacra. All of the tubercles about equal in size below the ambitus 

 but all except the middle series of each row of plates gradually disappear above the ambitus, the 

 latter alone continuing to the apical disk. 



