PLATYCRINID^. 727 



Arms from six to seven to each main division, or twelve to fonrteen to tlie 

 ray ; they stretch outward to the length of the palmars, and then bend 

 upward and inward, being roimded on the back, flattened at the sides, and 

 biserial from the second joint. 



Ventral disk fully twice as high as the dorsal cup, rather bulging ; the 

 plates large, heavy, and highly convex. Orals in contact laterally ; the 

 posterior one central in position and larger than tlie others, which are oblong 

 and pushed to the anterior side. The fixed covering plates of the ambulacra 

 extend out from the orals beyond the limits of the calyx to about one half 

 the length of the arms, and form together with the different orders of 

 brachials large, tapering, tubular trunks, from which the arms are given 

 off alternately at the sides. Interambulacral plates : 3, 2, 1, all large, but 

 especially the middle one of the first row. At the anal side the middle plate 

 is still larger, and is followed by nine or ten small plates, which take the 

 place of the second row of plates at the other sides, and form a small pro- 

 tuberance enclosing the anus. Above these plates there are two larger 

 ones, which, together with the smaller orals, form a ring around the pos- 

 terior one. Anal opening directed laterally. Stem elliptical, rapidly twist- 

 ing, its long diameter nearly three times the shorter one ; the joints slightly 

 increasing in width, and their proximal and distal faces provided with a well 

 defined ridge. 



Horizon and Locality. — Lower part of the Upper Burlington limestone, 

 Burlington, Iowa, and several places in Missouri. 



Ti/pes in the collection of Wachsmuth and Springer. 



Remarks. — This and the next species have the characters of the genus 

 less pronounced than in the typical forms, and they represent transition 

 forms in different degrees from Plati/crinus. 



