48 MESOZOIC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



? CiDARIS NAHALAKENSIS De Loriol. 



Cidaris nahalakensis De Loriol, 1887, Recueil zool. Suisse, vol. 4, pp. 388, 3S9, PL XVII, figs. 3^. 

 ICidaris nahalakensis Clark, 1893, U. S. Geol. Survey Bull. 97, p. 93. 



The species is based by P. de Loriol on fragments of spines obtained from the Selma chalk 

 ("Rotten limestone") of the Upper Cretaceous of Wahalak, Kemper County, Miss. Similar 

 spines have been recognized by the writer from other localities, but whether they represent an 

 independent species or one hitherto described is not clear. 



Genus I/EIOCIDARIS Desor. 



Leiocidaris hemigranosus (Shumard). 



Plate X, figures la-g; Plate XI, figures la-b. 



Cidaris hemigranosus Shumard. 1860, St. Louis Acad. Sci. Trans., vol. 1, p. 609. 



Cidaris hemigranosus Meek, 1864, Smithsonian Misc. Coll., vol. 7 (177), p. 2. 



Cidaris hemigranosus White, 1883, IT. S. Geol. and Geog. Survey Terr. Twelfth Ann. Rept., for 1878, p. 38, PI. XVIII, 



figs. 2a-b. 

 Leiocidaris hemigranosus Clark, 1891, Johns Hopkins Univ. Circ, vol; 10, No. 87, p. 75. 

 Leiocidaris hemigranosus Clark, 1893, U. S. Geol. Survey Bull. 97, pp. 38, 39, PL VII, figs. 2a-d; PL VIII. figs, la-b; 



PL IX, figs. la-c. 

 Leiocidaris hemigranosa Cragin, 1893, Geol. Survey Texas Fourth Ann. Rept. , p. 160, PL XLVI, fig. 4. 

 Leiocidaris hemigranosus Hill, 1901, U. S. Geol. Survey Twenty-first Ann. Rept., pt. 7, pp. 252, 266, 272. 



Determinative characters. — Test very large, subspherical. Ambulacra narrow, sinuous, 

 with six rows of granules in the middle. Poriferous avenues, wide, deeply depressed. Pores 

 of each pair united by clearly defined furrow. Interambulacra broad. Tubercles very large; 

 areola circular or slightly polygonal, depressed; boss smooth, mamelon large, perforated. 

 Miliary space covered with large scattered oval granules. Sutures clearly marked, depressed. 



Dimensions . — Diameter 90 millimeters; height 67 millimeters. 



Description. — The form, which was quite fully described by Shumard in 1860, but not 

 'fio-ured, was incorrectly referred to the genus Cidaris, from which it is separated by the presence 

 of furrows uniting the pores of each pair. In this respect it is a typical representative of the 

 o-enus Leiocidaris of Desor. A very good illustration was given by White in 1SS3 (report for 

 1878), although necessarily, from the size of the figure, the furrows connecting the pores could 

 not be very distinctly shown. Tins species is the largest echinoid known from the Texas Cre- 

 taceous, and with possibly a single exception the largest from the American Mesozoic. In one 

 unusually fine specimen the test is subspherical, although considerably broader and more flat- 

 tened on the oral than aboral side. 



The ambulacra are narrow and sinuous, with six rows of granules between the poriferous 

 avenues. The regular arrangement of the granules is not persistent, the number being increased 

 irregularly near the middle of the column and somewhat reduced toward the apical system and 

 peristome. The poriferous avenues are wide and deeply depressed, the pores of each pair being 

 united by a shallow furrow. 



The interambulacra are wide and the plates of massive size. The tubercles occupy the 

 center of the plates and are large and prominent. The areolas are deeply depressed, subcir- 

 cular or slightly polygonal, and occupy more than half of the greatest diameter of the plates. 

 On all the specimens examined the areolas show perfectly smooth surfaces, although Shumard 

 mentions the occurrence of radiating ridges. Toward the center each areola rises to form the 

 boss, which is provided marginally with a sharp smooth ridge. The mamelon is large, sub- 

 spherical, with a deep perforation upon its summit. The miliary space is wide and covered with 

 large, irregularly shaped and flattened granules, some round and some oval in form. Inter- 

 spersed among these are much smaller granules, winch increase in number along the margin of 

 the plates. The apical system and peristome are wanting. 



Related forms. — The species is readily separated from any hitherto described from American 

 or European strata. Its characters are well defined and distinctive. 



