PLATYCEINID^. 691 



small, slightly projecting near the arm bases. The interambulacral spaces 

 are formed of five plates, of which the middle one of the first row is twice 

 as large as the others, and hexagonal. The two or three upper stem joints 

 circular, the succeeding ones elliptic. 



Horizon and Locality. — Upper Burlington limestone ; Burlington, Iowa. 



Type in the Museum of Comparative Zoology. 



SCULPTUS GROUP. 



Basals and radials forming a deep cup ; the base high and distinctly con- 

 vex ; the plates ornamented by transverse lines of confluent nodes followino- 

 the margins of the plates. Eadial facets small ; arms moderately large. 



Platycrimis sculptus Hall. 

 Plate LXriII. Fig. 5, and Plate LXXV. Fig. 8, 9. 



185S. Hall ; Geol. Rep. Iowa, Vol. I., Part II., p. 536, Plate 8, Pig. 11. 



1881. W. and Sp. ; Eerision, Part II., p. 75 (Proceed. Acad. Nat. Sci. PliUa., p. 249). 



Sya. Plati/cdims roiundm S. A. Miller; Geol. Surv. Missouri, Bull. 4, p. 20, Plate 2, Pigs. 11 

 and 12. 



Dorsal cup of medium size, in very mature specimens higher than wide, 

 a little wider than high in small ones, growing proportionally higher with age; 

 the sides evenly convex from the stem upwards. Surface of plates oi-na- 

 niented with rows of confluent nodes or undulated ridges, the base being 

 covered by three to five rows, running parallel to the upper margins of the 

 plate ; five others radiate from the column to the itpper angles. Similar 

 rows, varying from three to six, according to the age of the specimen, fol- 

 low the margins of the radials, and three rows proceed from the facets to 

 the lower face; two of these directed diagonally to the outer angles, the 

 middle one, which is sometimes unrepresented, placed vertically. 



Base from one fourth to one third as high as the dorsal cup, basin- 

 shaped ; the median portions slightly truncated and a little excavated ; the 

 upper margins of the plate somewhat beveled, giving to the basi-radial 

 sutures a slight depression. Eadials quadrangular, the lower edge convex, 

 the sides very slightly spreading, the upper face but little sloping to the 

 angles. Facets small, semicircular, directed obliquely upwards, occupying 

 but one third the width of the j^lates, and only a small portion of their 

 height. Costals small, trigonal, sometimes narrower than the facets, so that 

 portions of the first distichals come in contact with the radials. First dis- 



