FOSSILS OF THE KEOKUK GROUP. 503 



twice as wide as high. Dorso-lateral pieces twice, as high 

 as Avide, presenting an irregular pentagonal outline, Avith 

 sloping sides above diverging at an angle of about 90 degs; 

 Vpper dorsal plate about half the size of the loAver, sub- 

 triangular, or nearly semicircular, slightly more than half 

 as long as Avide, and scarcely more than filling the notch 

 between the inner sloping sides of the dorso-lateral pieces. 

 Dorsal arm slender, rounded, and composed first of fiVe 

 pieces, the loAver of which is expanded below so as to be 

 nearly three-fourths as large as the upper dorsal piece, 

 AA'hile the succeeding pieces are narrow, and slightly longer 

 than wide, excepting the fifth one, which is a little Avider 

 than the others, pentagonal in form, and supports upon its 

 superior sloping sides two equal diA'isions, which are slen- 

 der, rounded, simple, and composed of pieces about tAvice 

 as long as wide. Lateral divisions of the rays (or perhaps, 

 more properly, supports of lateral arms,) composed of pieces 

 that are wider than long, and rapidly diminishing in size 

 from the lirst to the terminal one; of these, three can be 

 counted on one side, but there may have been one or two 

 more ; each supporting an erect arm, more slender than the 

 dorsal one, and dividing first on the third piece, the inner 

 division being smaller than the other and remaining simple, 

 while the larger one bifurcates again on the fourth piece, 

 the subdivisions being equal, and of the same size as the 

 inner branch at the first bifurcation. All the axillary pieces 

 are expanded and more protuberant at the upper end than 

 any of the others, though all of the other pieces are slightly 

 projecting at the upper end. (Ventral side unknoAAn.) 

 Column comparatively rather stout, or slightly thicker than 

 the dorsal arm beloAv its bifurcation ; composed, two or three 

 inches from the body, of round, nearly equal, moderately 

 thick pieces, but near the body shoAving a slight tendency 

 to become pentagonal, and apparently composed of more 

 irregular, somewhat roughened pieces. Surface rather dis- 

 tinctly granular, particularly on the dorsal side. 



