CRETACEOUS ECHINODEBMATA. 63 



The interambulaera bear two rows of primary tubercles that are similar to those of the ambu- 

 lacra, but larger and less numerous. The lower surface is covered by large mammillated granules 

 that can scarcely be distinguished from the tubercles, and also by numerous small ones. There 

 are few granules on the upper surface. The peristome is large, with slight incisions. 



Related forms. — The species is most closely related to C. speciosum, but in form and in 

 several details of structure is unlike it. The presence of granules between the pores of the 

 present species is apparently marked. 



Locality. — Wahalak, Kemper County, Miss. 



Geologic horizon. — Selma chalk, Upper Cretaceous. 



Collection. — P. de Loriol, Switzerland. 



Genua MICROPSIS Cotteau. 

 Micropsis lineatus Clark, n. sp. 

 Plate XXV, figures la-c. 



Determinative characters. — Test medium size, slightly polygonal in ambital outline, tumid 

 above, subhemispherical, slightly concave below. Apical system wanting. Ambulacra with 

 compound plates covered with two rows of primary tubercles perforate and crenulate. Inter- 

 ambulacra with six rows of primary tubercles, which rapidly decline in size above the ambitus. 

 Peristome small. 



Dimensions. — Diameter 25.25 millimeters; height 12 millimeters. 



Description. — The test is of medium size and slightly polygonal in ambital outline, with 

 tumid upper surface. It is subhemispherical and slightly concave on the lower surface. 



The ambulacra are narrow and covered with two vertical rows of perforate and crenulate 

 primary tubercles. The poriferous zones are not sharply denned. The plates are compound, 

 with a low primary adorally. 



The interambulaera have six rows of primary tubercles which decline rapidly in size above 

 the ambitus. 



The peristome is small and with slight branchial incisions. 



Locality. — Cut on Bonham road, southeast edge of Denison, Tex. 



Geologic horizon. — Grayson marl member of the Denison formation, of Washita group, 

 Comanche series, Cretaceous. 



Collection. — U. S. National Museum (31198). 



Suborder BCHININA. 

 Family TRIPLECHINID.E. 



Genus PSAMMECHINTTS Agassiz. 



Psammechinus CINGULATUS Clark. 



Plate XXII, figures 2a-i. 



Psammechinus cingulatus Clark, 1891, Johns Hopkins Univ. Circ, vol. 10, No. 87, p. 76. 

 Psammechinus cingulatus Clark, 1893, Johns Hopkins Univ. Circ, vol. 12, No. 103, p. 51. 

 Psammechinus cingulatus Clark, 1893, U. S. Geol. Survey Bull. 97, p. 55, PL XX, figs. la-i. 

 Psammechinus cingulatus Johnson, 1905, Acad. Nat. Sci. Philadelphia Proc, vol. 57, p. 6. 



Psammechinus cingulatus Welter, 1907, Cretaceous paleontology of New Jersey: Geol. Survey New Jersey, pp. 

 288-289, PL X, figs. 1-9. 



Determinative characters. — -Test small, hemispherical, with circular outline; sides inflated; 

 upper surface elevated; base flattened and concave. Ambulacra wide, with two rows of 

 primary tubercles, sixteen or seventeen in each series; poriferous zones depressed, triserial. 

 Interambulaera about one and one-half times the width of the ambulacra; plates covered with 

 numerous uncrenulatc and imperforate tubercles. Peristome large, with ten deep incisions. 



dimensions.— Diameter 22 millimeters; height 12.5 millimeters. 



Description.— This delicate form, of which two very imperfect specimens were examined by 

 the -writer, is hemispherical in shape, with circular circumference, inflated sides, elevated upper 



