438 THE CRmOIDEA CAMEEATA OF NORTH AMERICA. 



of the costals — -whence it spreads abruptly to the arm bases. Plates heavy, 

 their middle portions generally prominent, their surfaces smooth, and the 

 suture lines not impressed. 



Base once and a half as wide as long, truncated at the bottom, and slightly 

 spreading outward from the lower edge ; column facet deeply impre.ssed. 

 Kadials large, generally longer than wide, often decidedly so ; rising verti- 

 cally. Costals small ; the first quadrangular, not more than one fourth the 

 size of the radials ; the second a little wider but not longer than the first. 

 Distichals 2 X 10, of the same size as the costals. Each one is followed by 

 two palniai's, which either form a continuous series around the calyx, or are 

 separated between the rays by a small interdistichal plate. Arm bases pro- 

 jecting ; the arm openings directed obliquely upwards. Arms four to the 

 ray, short, rather flat and heavy. They are composed of two rows of .short 

 pieces alternately arranged, which are produced laterally into slender, sharply 

 pointed si^ines extending out from the sides of the arms. Interbrachial plates 

 1, 2, 2, followed sometimes by another narrow plate. Anal plate as large 

 as the radials ; supporting three smaller plates in the first range, and three 

 in the second, which are frequently followed by one or two other plates. 

 Tegmen conical, slightly bulging ; composed of large convex or nodose 

 pieces, which are continued to the anal tube. Orals excentric, and a little 

 larger than the surrounding plates, especially the posterior one. The radial 

 dome plates are also readily distinguished by their larger size and greater 

 convexity. Anal tube almost central, very long and heavy, often rising to 

 three times the length of the arms ; it gradually tapers, and has at the upper 

 end a minute opening. Column long, rather stout. It has been observed to 

 the length of twenty inches, to which it neither increases nor decreases 

 in size, and the length of the joints remains unaltered; but the edges of 

 the upper ones are slightly convex, while the lower joints are strictly 

 cylindrical. 



Horizon and Locality. — One of the leading fossils of the Upper Burling- 

 ton limestone, and found wherever that is exposed throughout Iowa, Illinois, 

 and Missouri. 



Remarks. — A. jjpiformis, var. mdis, is in our opinion identical with this 

 species. The original specimen was said to come from the Kinderhook group 

 of Marion Co., Ills. ; but we doubt it, for it is a true Upper Burlington type. 



