286 PROCEEDINGS OF THE ACADEMY OP [1885. 



Why not the first plate also ? We seriously doubt if those plates 

 enclose the perisome as the interradials in Actinocrinus, or were 

 covered by perisome as in Cyathocrinus^ and hence believe they 

 are not calyx but perisomic plates, which, like the smaller pieces 

 of Extracrinus, united the lower arm divisions. We take the 

 same view of the so-called interradials and interaxillaries of 

 Uintacrinus, which merely attained the outer form of calyx 

 pieces, hut are true disk plates, and on approaching the ventral 

 si le passed into anambulacral pieces instead of harboring or sup- 

 porting a perisome. The case is altogether different in Thaumato- 

 crinus, in which the interradials are placed within the ring of first 

 radials, and as such form, like the anal plate, a primitive part of 

 the calyx. The Crotalocrinidse present a different perisomatic 

 arrangement from the Actinocrinidae. The interradials frequently 

 commence in the equatorial zone, and extend over the whole 

 ventral surface, even oral plate and proximals being subtegminal. 

 Their perisome, which was figured by Angelin in Crotalocrinns 

 rugosus (Icon. Crin. Suec, PI. xvii, fig. 3 a), is composed ex- 

 clusively of covering plates. The proximals are long and narrow, 

 and abut with their outer edges against the deflected upper ends 

 of two radials. leaving radially live angular spaces, which are 

 occupied by the ambulacra. These ambulacra, of which the 

 covering plates are visible, bifurcate like those of other groups, 

 but their subdivisions, in place of being separated by anambu- 

 lacral plates, join each other laterally, and, together with the 

 summit plates, fill the entire ventral surface. The total absence 

 of anambulacral pieces in this genus is a most remarkable feature, 

 hut may perhaps be explained by the presence of hydrospires. 

 There are, however, no spiracles nor pores through any of the 

 plates, except along the anal tube, which is perforated along its 

 walls. 



The vault of the Crotalocrinida' extends quite a distance into 

 the free rays, us shown by Midler's and Angelin's figures (Icongr., 

 PI. 0, figs. 6 ami 7, also PI. 25, figs. 15 and 25, and Akademie der 

 Wissenschaften, 1853, PI. 13, fig. 10). That those plates are not 

 ambulacral pieces is proved by the tact, that they cover the 

 Saumplatten, and have a different style of ornamentation. Those 

 figures further prove, that the ventral covering was pliable, or the 

 arms could not have assumed that horizontal position, and be 

 folded in other specimens. This is of some importance as dem- 



