50 MESOZOIC ECHINODEEMATA OF THE UNITED STATES. [bull-OT. 



Belated forms. — This species is closely allied to or perhaps identical 

 with the form described by D'Orbi guy in 1842 from the United States of 

 Colombia under the name of Echinus Bollvaril., and later recognized by 

 Gabb in a collection obtained fi'om Peru. As the original description 

 and figures leave some doubt as to the determination of the species, the 

 term texanum is retained here. 



Locality and geological horizon. — This form is found in the Fredericks- 

 burg formation of the Comanche series (lower Cretaceous) of Texas. It 

 is from the Conmnehe Peak beds, and is reported from Fredericks- 

 burg and near the head of the San Saba river. 



Collections. — U. S. National Museum ; Boston Society of Natural His- 

 tory; Texas Geological Survey; University of Iowa; Prof. Kobt. T. Hill. 



DiPLOPODiA HiLLi Clark. 

 Plate XVI, Figs. 2a^g. 



Pseudodiadema RiUi Clark, 1891. Jolius Hopkins University Circulars, No. 87, p. 76. 



Determinative characters. — Test subpentagonal, depressed ; sides in- 

 flated; upper and lower surfaces flattened. Ambulacra prominent, 

 straight, with two rows of xnimary tubercles, twelve or thirteen in each 

 row ; poriferous zones narrow, with slight increase in width toward the 

 apical disk ; pores in single pairs below but bigeminal on the upper 

 surface. Interambulacra with two rows of primary tubercles of equal 

 size with those of the ambulacra. Mouth opening narrow, circular. 

 Discal opening broad, pentagonal. 



Di^nensions. — Transverse diameter, f inch; height, -^^ inch. 



Description. — This rare Dii)lopodia has a subpentagonal outline, the 

 projecting ambulacra .occupying the angles. It is depressed on both 

 the upper and lower surfaces, the latter becoming somewhat concave 

 toward the mouth opening. The sides are moderately inflated. 



The ambulacra are broad, very prominent, and furnished with two 

 rows of large tubercles, twelve or thirteen in each row>that decrease 

 rapidly from tlie ambitus toward tlie apical disk and peristome. The 

 poriferous zones are narrow and lanceolate. The pores are arranged 

 in single pairs on the lower surface, but from the ambitus to the apical 

 disk are increased slightly in number, and near the apical disk become 

 bigeminal (PI. xvi, Fig. 2(7). The interambulacral areas are narrow 

 and appear flattened, as compared with the slightly i)rojecting ambu- 

 lacra. They are provided Avith two rows of primary tubercles of equal 

 size with those of the ambulacra (PI. xvi, Fig. 2/'). There are ten or 

 eleven in each row. The areolas are narrow, circular, and elevated, the 

 bosses prominent and distinctly crenulated, and the mamelons deeply 

 jjerforated. The tubercles are somewhat widely separated and the inter- 

 vening space is eovered with small secondary tubercles and sparsely 

 scattered granules. 



The mouth opening is broad, depressed, and circular. It is divided 



