CRETACEOUS ECHINODERMATA. 53 



The miliary space is wide, very nearly smooth in the upper portions of the column, and sparsely 

 covered with granules toward the peristome. 



The apical system is large and compact, the elongated genital plates giving it a star-shaped 

 appearance. The well-defined ocular plates, wedged into the interspaces, make the entire out- 

 line subpentagonal. Each genital plate is angular at the outer extremity and perforated. On 

 the inner edge of the same there is a semicircular depression, in the center of which is a small 

 mammillated tubercle. 



The peristome is large, covering quite one-half the diameter of the base. It is divided into 

 ten lobes by ten shallow incisions. 



Related forms. — The species shows many points of similarity to Echinus patagonensis, 

 described by D'Orbigny in 1842, from the Tertiary of southern Patagonia. The margin of 

 D'Orbigny's species is sharper and its apical system of somewhat different shape; moreover, if 

 properly referred to the Tertiary, it is undoubtedly a different form. Echinus andinus, from the 

 desert of Atacama, described by Philippi in I860, is likewise closely allied to 6?. zitteli, although 

 more conical and presenting a different surface decoration of the plates. Goniopygus major, 

 from the Cenomanian of Europe, is similar in many particulars, although the pores are not 

 increased around its peristome as in G. zitteli, and its apical system is more compact. 



Locality. — Round Rock (type) and Spanish Oak branch, Williamson County, Tex. 



Geologic horizon. — Edwards limestone of Fredericksburg group, Comanche series, Cretaceous. 



Collections. — U. S. National Museum (12234); Texas Geological Survey. 

 Genus LEPTARBACIA Clark, n. gen. 



Test small, circular in ambital outline, subhemispherical, upper surface depressed, lower 

 surface flat. Ambulacra! plates are simple primaries near the apical system and compound at 

 the ambitus. Compound plates consist largely of one primary and three demiplates, two above 

 and one below. Interambulacra with large plates and with six rows of small perforated and 

 crenulated tubercles on lower surface which become reduced. Peristome of medium size. 

 Periproct small. . 



Leptarbacia argutus Clark, n. sp. 



Plate XVI, figures la-f. 



Determinative characters. — Test small, depressed above, flat below, apical system of medium 

 size, periproct nearly circular. Ambulacra straight, rather wide at ambitus and on lower sur- 

 face, plates compound excejDt near apical system, generally composed of one primary and three 

 demiplates, some plates with one primary and two demiplates. Interambulacra with long bare 

 median areas on upper surface, approaching ambitus, and on lower surface six rows of small per- 

 forated and crenulated tubercles. 



Dimensions. — Diameter 20 millimeters; height 12 millimeters. 



Description. — The single specimen of this species is small, with depressed upper surface 

 and nearly flat below. It is somewhat depressed near the peristome. 



The ambulacra are narrow, increasing in width from the apical system to the ambitus, 

 beyond which they decrease in width toward the peristome. There is a double row of about 

 thirty tubercles in each area, which become quite indistinct toward the apical disk. The pores 

 are arranged in an irregular simple series in each poriferous zone. 



The interambulacra are about three times the width of the ambulacra, and each contains 

 from the ambitus to the peristome six rows of small perforated and crenulated tubercles, the 

 central row of tubercles of each half continuing to the apical system, although gradually becoming 

 less distinct as the latter is approached. The miliary space is covered with small granules. 



The apical system is small, the genital plates being somewhat larger than the ocular, but 

 rarely projecting farther than the latter. The periproct is small and slightly subcentral. 



The peristome is apparently of medium size but is not well enough preserved to show its 

 characteristic features. 



Locality. — Fort Worth, Tex. 



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



Collection. — Johns Hopkins University (T 3004). 



