52 MESOZOIC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



Suborder ARBACINA. 

 Family HEMICIDARIDvE. 



Genus HYPODIADEMA Desor. 

 Hypodiadema elegans Clark, n. sp. 

 Plate XIV, figures 2a-b. 



Determinative characters. — Test small, moderately inflated, slightly tumid above the ambitus. 

 Ambulacra narrow, nearly straight, with small perforate, crenulate tubercles diminishing in size 

 from the ambitus aborally. Interambulacra broad, with moderately large tubercles perforate 

 and crenulate. Apical system prominent, genital plates large, forming a periproctal ring; ocular 

 plates small, elevated. 



Dimensions. — Diameter 13.5 millimeters. 



Description. — Only a single specimen of this species has been found. The abactinal and 

 central portions of the test are well preserved, but the actinal surface is covered. The delicately 

 sculptured test is small, somewhat inflated above. The ambulacra are nearly straight and nar- 

 row. The small primary tubercles at the ambitus are perf orate and crenulate, but diminish in size 

 toward the apical system. 



The interambulacra are broad, the primary turbercles large, crenulate, and perforate, the 

 depressed areolas being surrounded by a circle of granules. The miliary space is narrow, except 

 toward the apical system, where it is large on the abutting plates. The apical system is elevated 

 and pronounced. The large genitals which form the periproctal ring are perforated, the madre- 

 porite having two large openings. The oculars are small and elevated. 



Locality. — Junction of Muddy and Clear forks of Little Missouri River, 3 miles west of Mur- 

 freesboro, Pike County, Ark. 



Geologic horizon. — Trinity sand of the Comanche series, Cretaceous. 



Collection. — U. S. National Museum (31196). 



Genus GONIOPYGUS Agassiz. 



Goniopygtjs zitteli Clark. 



Plate XIV, figures 3a-c; Plate XV, figures la-h. 



Goniopygus zittelli Clark, 1891, Johns Hopkins Univ. Circ, vol. 10, No. 87, p. 76. 



Goniopygus zittelli Clark, 1893, Johns Hopkins Univ. Circ, vol. 12, No. 103, p. 51. 



Goniopygus zittelli Clark, 1893, U. S. Geol. Survey Bull. 97, pp. 53, 54, PL XVIII, figs. 2a-d; PI. XIX, figs. la-e. 



Goniopygus zitteli Cragin, 1893, Geol. Survey Texas Fourth Ann. Kept., p. 155. 



Determinative characters. — Test subcorneal, circular in outline ; upper surface elevated ; base 

 flattened. Ambulacra broad, with a double row of alternating tubercles, 18 to 20 in each series; 

 poriferous zones, with a double row of pores, multiplied at the peristome. Interambulacra 

 broader than the ambulacra, with a double- row of primary tubercles, 10 to 12 in each row. 

 Apical system prominent, compact. Peristome large, with shallow incisions. 



Dimensions. — Diameter 25 to 38 millimeters; height 15 to 25 millimeters. 



Description. — All specimens examined by the writer have a distinctly subcorneal form that is 

 highly characteristic. The upper surface is very much elevated, while the lower is flat and 

 slightly depressed around the peristome. 



The ambulacra are broad, increasing regularly in width from the apical disk to the ambitus, 

 beyond which they decrease toward the peristome. There is a double row of 18 to 20 

 tubercles in each area that rapidly increase in size from the apical system downward. The pores 

 are arranged in a double row on each poriferous zone, except in the vicinity of the peristome, where 

 they become suddenly increased in number. 



The interambulacra are about one and one-half times the width of the ambulacra, and each 

 contains a double row of large tubercles, 10 to 12 in each series. These tubercles are con- 

 siderably larger than those of the ambulacra! area, and have wide circular areolas, smooth bosses, 

 and imperforate mamelons. On the base the tubercles of all the areas are of nearly equal size. 



