10 



All of the interradial ureas connect witli tlie vault. There 

 ai-e four plates in each regular interradial area, one in the first 

 range, two in the second, and one in the third, which sep- 

 arates the radial series and unites with llic ])latcs of Ihc vault. 

 There are fourteen plates in the azygoiis area. Tlu; tii-sl om; 

 is in line with the first primary radials and of al;out the same 

 size: it is followed Ijy three plates in llic srcoiid range, six 

 plates in the third range, and lour in the fowith range, two of 

 which unite with the plates of Ilic \anll. One of tiie four 

 plates, in tlie last range, is \n the left- and another is intei- 

 calated between the lower ]iart of tin- two larger plates thai 

 unite with the plates of the vauH 



The vault is depressed, convt^x and coxennl with suiali, coi; 

 V(!X, polygonal plates and has a subcential azygous oritice 

 slightly elevated above the gi-eatest convexity of the vault, 

 but it could hardly be i'egard(>d as a jn-oboscis. No o\;irian 

 pores can be discovered. 



This is a very peculiar species and so different from all 

 other eighteen-armed species that no comparison witli any of 

 them is neces.sary. 



Found by F. A. Sam])son. in tlie l\<'ol\ul\ (irou)). at Hoon- 

 villc. Missouri, and now in his collection. 



I'.ATltCK'INt S SHJNATIS, U. S]i. 



Plate I, Fig. in, nzyf/niis side: Fi<i. 17, opposUr view. 



Species mediuoT or below medium size, biturbiuale. Calyx 

 broadly truncated, obpyramidal, about twice as wide as high. 

 Radial ridges angular and well defined. Interradial areas llat 

 tened. Ambulacral openings directed a little ;,l.ove a horizon- 

 tal line. 



Basals form an hexagonal disc that is (^x])and(-d in I'im around 

 the ba.se of the calyx. Diamelei' a little greater than the 

 diameter of the column, and height about equal to the thick- 

 ness of the plates. Shallow concave de])ression for the at- 

 tachment of the column. P^irst primary radials a little wider 

 than long, longitudinally angular in the middle i)art, three 

 hexagonal, two heptagonal. Second i)rimai'y radials short, 

 quadi-angnlar. three or foui- times as wide as long. Third 



