58 MESOZOIC ECHINODERMATA OF THE UNITED STATES, [buia.97. 



lEEEGULAKES. 

 ECHINOCONID^E. 



Test circular, elliptical, or pentagonal. Ambnlacra narrow; porifer- 

 ous zones uniij^eniinal, seldom biiicniinal. Interanibnlacra broad ; surface 

 covered witli small, perforated, and crenulated tubercles, that are larger 

 on the base. Mouth opening- central, circular, or decagonal, with jaws 

 and distinct incisions. Apical disk central, com])osed of ten plates. 

 Anal opening between apical disk and mouth opening. 



HOLECTYPUS Desor. 



HoLECTYPtis PLANATUS Eoemer. 



Plate XXIII, Fig. 2a-f. 



Holectypus planatus Eoemer, 1849. Texas, etc., p. 393. 



Holeeli/jms planatus Eoemer, 1852. Die Kreidebiklungeu von Texas, p. 84, pi. 10, 



Fig. 2. 

 Holeciypus 2)l(inafus Sbumard, 1852. Eept. Expl. Bed Eiver of La.,p, 211. 

 Hplectypus planattis Giebel, 1853. Jabresbcr, d. Naturw. Ver. in Halle, p. .373. 

 ' Holed ypus pJitnns Giebel, 1853, ibid, 

 HoU'ciypus planatus Conrad, 1857. U, S. and Mes. Bonnd. Surv. Eept. vol. 1, pt. 2, 



p. 145, pi. 1, Fig. 4. 

 HolecUjpus planatus Desor, 1858. Synop. des Ecbiuides fossiles, p. 174. 

 Hohctypns planatus Gabb, 1860. Cat. Invert. Fossils, Cretaceous, ]>. 18. 

 Holectygus planatus Meek, 1864. Sniitb. Misc. Coll., vol. 7 (177)', p. 2. 

 Holectypus planatus Clark, 1891. .Johns Hopkins University Circulars, No. 87, p. 76. 



Determinative characters. — Test subcircular, subconical, flattened on 

 the under surface. Ambulacra narrow, straight, and somewhat lanceo- 

 late, with six irregular rows of tubercles; poriferous zones straiglit, 

 narrow, unigeminal. Interanibnlacra wide, plates numerous and nar- 

 row, each with a nearly horizontal row of small tubercles. Apical 

 disk small, the madreporite large. Mouth opening small, subcircular. 

 Anal opening large. 



Dimensions. — Height, f inch; transverse diameter, If inches. 



DescriiMon. — This very common form, from the Cretaceous of Texas, 

 was first reported by Eoemer in "Texas," etc., in 1849, and subsequently, 

 in 1852, more accurately defined and figured. Although wide differences 

 in outline and in the size and arrangements of the tubercles occur 

 they appear to be rather individual than specific. The majority of the 

 specimens are subcircular; in the case of some, however, distinctly cir- 

 cular, while in that of others clearly pentagonal. The ambitus in some 

 individuals is sharp, in others, rounded, while the ui)per surface is dis- 

 tinctly elevated at the center with slightly tumid sides. The under 

 surface is flat and slightly depressed in the vicinity of the mouth opening. 



The ambulacral areas are narrow, straight, increasing in width toward 

 the ambitus and somewhat lanceolate in form. The poriferous zones 

 are narrow, the pores small and unigeminal. Six rows of small tuber- 



