CLARK.) " UINTACRINUS SOC'IALIS. 23 



lary, bearing two large distichals that come in contact only nlong a 

 portion of their lateral margins ( I'l. i, Fig. Ic). Sncceeding the primary 

 distichals are four others of large size and varying shape, bej-ond which 

 the plates rapidly decrease in size and change to true brachials (PI. i. 

 Fig. Ic). 



In tlie angle between the larger distichals of each radial series are 

 two large interdistichal plates. The lower of these comes in contact 

 with the primary distichals in the angle formed by the upi)er i>ortions 

 of the lateral margins of those i^lates. It likewise touches the sides of 

 the second and a portion of the third distichal. The upper interdis- 

 tichal rests with horizontal edge upon the lower, and is embraced be- 

 tween the upper portion of the third distichals and the lower i^ortion 

 of the fourth, ending above with angular margin between the modified 

 plates that branch from the fourth distichals, to be presently referred to. 



The second distichals are axillary and support interradially rows of 

 plates, the first of ea(;h row separated from one another by the inter- 

 radials (PI. i, Fig. 1«). The first plates are smaller than the second of 

 the same row, which meet with a vertical suture and slightly overlap, 

 a character which becomes more x>ronouuced higher in the series, so 

 that true alternation results. Beyond the third i)late a rapid'reduction 

 in size of plates takes place. Ten to twelve plates have been recog- 

 nized in each row. 



From the fourth distichals, which are likewise axillary, similar rows 

 diverge radially (PI. i. Fig. Ic). From the fifth distichals are other 

 rows interradial in position, while the seventh again bear radial 

 branches. These rows gradually grow smaller and as the brachials 

 become movable appear as pinnules. The lower rows beccmie incor- 

 porated in the calyx, and as they are immovable have become modified 

 for the position they occupy. 



The interradials are eight, nine, or more in iniinber and form an ele- 

 vated, shield-like area, each plate likewise showing a slightly convex 

 surface (PI. i. Fig. la). The arrangement of the plates does not 

 vary; seven in an oval band inclose the eighth, or eighth and ninth, 

 according to the num])er of interradials. The lower interradial occupies 

 the angle between the upi)er i)ortions of the lateral margins of the 

 primary radials and likewise separates the secondary radials. The two 

 ui)per interradials occupy the angle formed by the branches from the 

 second distichals and the distichals tliemsehes. The four other inter- 

 radials of the outer band lie two on each side between the lower inter- 

 ladial and the upper pair of plates. The lower two of these four are 

 in contact, while the upper two are separated by the inner plate or 

 plates. In contact witli tlie four plates are tlie sin-ondary and tertiary 

 radials and the first and second distichals. The inner plate is more fre- 

 quently replaced by two smaller ones irregular in outline and jiosition. 



The brachial plates are tlie continuation of the distichals and form 

 ten simple arms. They gradually assume a half-round, horseshoe- 



