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476 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
placed between the two interbrachials. The plates of the ventral disk are 
rather small, the posterior oral excepted, which is quite large, tuberculiform, 
and central in position. The anus is placed a little above the arm regions; it 
is surrounded by numerous small plates, and opens out laterally. The am- 
bulacra apparently are roofed by small covering pieces. 
Horizon and Locality. — Lower Burlington limestone; Burlington, Iowa. 
Type in the Museum of Comparative Zodlogy. 
Aorocrinus subaculeatus (Hatt). 
Plate XLV. Figs. 7a, 0. 
1858. <Actinocrinus subaculeatus —HauL; Geol. Rep. Iowa, Vol. I., Part IT., p. 570, Plate 10, Figs. 2a, b. 
1873. Dorycrinus subaculeatus —MuzEx and WortuEn; Geol. Rep. Illinois, Vol. V., p. 380. 
1881. Dorycrinus subaculeatus — W. and Sp.; Revision Paleocr., Part IT., p. 180(Proceed. Acad. Nat. Sci. 
Phila, p. 354). 
A small and very rare species. Dorsal cup symmetrically semiglobose ; 
the tegmen abruptly conical, with an acute spiniform plate at the top. 
Plates delicate, flat (their natural curvature excepted) and perfectly smooth ; 
the suture lines obscure. — 
Basals in form of an inverted cup, very small, and frequently not visible 
in a side view. Radials almost as long as wide at the upper end, the lower 
end considerably narrower and bending inward. Both costals together equal 
to the size of the radials; the first quadrangular, fully twice as wide as long; 
the second pentangular and wider as well as longer. Distichals 1 X 2; those 
of the posterior rays axillary, and followed by two single very small pal- 
mars. In the antero-lateral rays only that next to the posterior ray is axil- 
lary, the other subquadrangular; the former supporting two small palmars 
and two arms, the latter a single arm. The same structure prevails in the 
anterior ray, in which the distichal to the right supports the two arms. The 
arm-bearing plates somewhat projecting. Arm openings directed laterally, 
arranged in groups, the spaces between the rays depressed, that of the anal 
side deeper and wider. Arms fifteen; their structure not known. The first 
anal plate is longer than the radials and fully as wide ; it supports three large 
plates in the first range, and three much smaller ones in the second. The 
plates above are small and irregularly arranged, forming a flattened area, 
which extends to the posterior oral. The interbrachials of the other areas 
consist of three plates in the dorsal cup, and two between the arms, which 
latter insensibly connect with the interdistichals. The anus is placed half 
