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 Localiti/. — Lower Burlington limestone ; Burlington, Iowa. 



I'yj^e in the Museum of Comparative Zoology. 



Aorocrinus subaculeatus (Hall). 

 PMe XLV. Figs. 7a, b. 



1858. Actinocrimis siibaculeaius — Hall ; Geol. Hep. Iowa, Vol. I., Part II., p. 570, Plate 10, Pigs. 2a, b. 

 1873. Dori/crinus mibaeuleatus — Meek and Woktuen; Geol. Rep. Illinois, Vol. V., p. 380. 

 1881. Borycriims subaculeatus — W. and Sp. ; Revision Palseocr., Part II., p. lSO(Proceed. Acad. Nat. Sci. 

 Pbila., p. 35i). 



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 cnp, very small, and frequently not visible 

 in a side view. Eadials 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 1x2; 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 one's 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 



