PALAEONTOLOGY OF IOWA. 621 



JiMg. 1. View of a very fine specimen preserving a portion of the column, and the arms 

 to the extent of three and a half inches, with about eighty and in one about 

 ninety joints on each side. The entire number was probably more than one 

 hundred joints. 



This splendid specimen, from the Cabinet of Mr. H. A. WORTHEN, is the 

 only one known, so far as I am aware, which preserves the arms. 



Geological position and localities. In the Keokuk limestone : Keokuk, 

 Iowa, and Warsaw, Illinois. 



Agaricocrimts wliitfieldi ( n. s.). 



BODY depressed-hemispheric above ; the base deeply con- 

 cave for the reception of the column, the concavity reaching 

 to the brachial plates. Basal plates not extending beyond the 

 circumference of the column. First radials short, much wider 

 than long. Second radials of the anterior and antero-lateral 

 rays regularly quadrangular ; of those of the postero-lateral 

 rays, one is oblique and unequally quadrangular, and the 

 other has its upper acute angle truncated by one of the 

 interradial plates. Of the five third radial plates, two are 

 hexagonal and three pentagonal. First brachial plates very 

 large, and succeeded by two central somewhat wedgeform 

 subaxillary plates, and a broad lateral one on each side ; and 

 upon these plates the arms originate in two double series of 

 plates. First interradial plate elongate, somewhat irregular- 

 ly octagonal, reaching as high as the third radial or to the 

 brachial plates, and supporting on its upper sides one or two 

 narrow elongate plates which extend between the arms, and 

 reach to the summit of the arm-bases. First anal plate longer 

 than the first radials, supporting on its upper truncated side 

 an elongate hexagonal plate ; its upper oblique sides, and 

 the oblique sides of the adjacent radials, supporting two 

 elongate irregularly octagonal plates, which reach about as 

 high as the centre of the arm-bases : above these, a series 

 of five plates, one central hexagonal, and two elongate sub- 

 hexagonal on each side, while between these and the aper- 

 ture are three series of smaller plates. 



DOME composed of numerous small plates, a larger de- 



