CRETACEOUS ECHINODERMATA. 77 



Cassidulus subquadratus Conrad. 



Plate XXXI, figures 2a-g. 



Cassidulus subquadratus Conrad, 1860, Acad. Nat. Sci. Pliiladelphia Jour. , 2d ser., vol. 4, p. 291, PI. XL VII, fig. 10. 



Cassidulus subquadratus A. Agassiz, 1883, Mus. Comp. Zool. Mem., vol. 10, p. 90. 



Cassidulus subquadratus Clark, 1891, Johns Hopkins Univ. Circ, vol. 10, No. 87, p. 77. 



Cassidulus subquadratus Clark, 1893, Johns Hopkins Univ. Circ, vol. 12, No. 103, p. 52. 



Cassidulus subquadratus Clark, 1893, U. S. Geol. Survey Bull. 97, pp. 70, 71, PI. XXXI, figs. la-h. 



Cassidulus subquadratus Johnson, 1905, Acad. Nat. Sci. Philadelphia Proa, vol. 57, p. 7. 



Cassidulus subquadratus Slocum, 1909, Field Mus. Nat. Hist. Pub. 134, Geol. ser., vol. 4, No. 1, pp. 5, 6. 



Determinative characters. — Test subquadrate to subpentagonal, hemispherical; sides equally 

 declining, upper surface convex; lower surface concave; posterior margin truncated and ros- 

 trated. Poriferous zones broadly petaloidal on upper two-thirds of the dorsal surface; sharply 

 contracted at lower margin of petaloidal areas. Peristome small, pentagonal, with well-defined 

 flosceUe. Apical system large, slightly anterior to the center. Periproct round, in deep sulcus. 



Dimensions. — Length 56.5 millimeters; width 53 milhmeters, height 28 milhmeters. 



Description. — This remarkably symmetrical Cassidulus has a nearly hemispherical form. 

 In outline it is nearer subpentagonal than subquadrate, owing to the rostrated character of the 

 posterior margin, which is at the same time truncated. The anterior margin is rounded. The 

 lateral edges are nearly parallel, though somewhat more approximated anteriorly. The apex is 

 slightly forward of the center. 



The ambulacra are moderately wide in the petaloidal portion, narrow beyond. The porif- 

 erous zones are composed in the petaloidal portion of an inner row of smaU, nearly circular pores 

 and of an outer row of oblique slitlike openings. The pores of each pair are united by a shallow 

 furrow. Beyond the petaloidal areas the narrow plates of those portions give place to broad 

 plates, each with a pair of small, round pores in the lower and outer corner. In the vicinity of 

 the peristome the poriferous zones expand and the pores increase in size, though apparently 

 reduced in number. The last pair of plates are much contracted. 



The interambulacra are wide. The plates are covered with minute tubercles with depressed 

 areolas, which become larger and more prominent on the lower surface. 



The apical system is large, composed of four perforated genital and five ocular plates. The 

 fifth genital is apparently obsolete. 



The peristome is small, pentagonal, and surrounded by a well-developed floscelle. The 

 periproct is round and is situated high above the margin, in a deep sulcus, that becomes narrower 

 and shallower toward the ambitus. 



Related forms. — This species is most closely related to Cassidulus suhconicus, but the latter 

 is much more elevated and has its periproct- situated higher and in a less depressed sulcus. 



Locality. — Near Holly Springs, Miss. 



Geologic horizon. — Ripley formation, Upper Cretaceous. 



Collection, — U. S. National Museum (31200). 



Cassidulus subconicus Clark. 



Plate XXXII, figures la-k. 



Cassidulus subconicus Clark, 1891, Johns Hopkins Univ. Circ, vol. 10, No. 87, p. 76. 

 Cassidulus subcotiicus Clark, 1893, Johns Hopkins Univ. Circ, vol. 12, No. 103, p. 52. 

 Cassidulus subconicus Clark, 1893, U. S. Geol. Survey Bull. 97, pp. 71, 72, PI. XXXII, figs. la-k. 



Determinative characters. — Test subcorneal; margin subpentagonal, truncated and rostrated 

 posteriorly; apex anteriorly placed. Ambulacra broad in petaloidal areas. Apical system 

 large, anteriorly situated. Peristome subcircular, with very prominent floscelle. Periproct 

 large, situated high on upper surface, in shallow sulcus. 



Dimensions. — Length 47 milhmeters; width 43 milhmeters; height 27.5 millimeters. 



Description. — The test is subcorneal. In marginal outline it is subpentagonal, the pos- 

 terior border more or less rostrated and clearly truncated. The apex is slightly forward of the 

 center, giving a somewhat sharper slope to the anterior portion of the test than to the posterior. 

 39800°— 15 



