344 PALEONTOLOGY OF ILLINOIS. 



The genius Acrocrinus departs from most Palseocrinoidea in two 

 important points. The plates of the calyx, which in all species 

 with a large number of plates decrease in size from the basals to 

 the top of the calyx, in Acrocrinus decidedly increase in the same 

 direction. Another striking departure is that the radials are not 

 connected with the basals, and partly not even among each other, 

 but are separated by several rings of plates, which in their position 

 are partly radial, partly interradial, and which have apparently no 

 representation in other genera of the Palseocrinoidea.t 



The specimen under consideration is composed of 86 plates, some 

 of them extremely minute. There are two comparatively large basals, 

 equal in size, the suture passing from the anterior to the posterior 

 side, which together form a concavity within the truncate part of 

 the calyx. The basal disk is surrounded by a ring of twelve very 

 small triangular pieces, and these in turn are succeeded by a second 

 series of seven plates each, arranged like the former, occupying the 

 azygous side, and are separated, in place by radials, by a row of 

 four hexagonal special anal plates, which, with the exception that 

 the upper side of the upper plate is not excavated, have exactly the 

 form and size of the four radial plates at the anterior ray. This 

 ray has exceptionally four radials, hexagonal like the anals, which, 

 with their truncate side, are connected among each other, and with 

 the heptagonal piece of the second ring heretofore described. The 

 four lateral rays not only consist of but three plates, but these 

 radials have also a very different form, and are partly disconnected. 

 The first is hexagonal, with upper and lower sides angular, the 

 second pentangular, angular below. Only in a single ray of the 

 specimen do the angles of the two plates touch each other; in the 

 four others they are separated by plates from different interradial 

 areas, which join here, there suture forming a line between the 

 angles of the two radials. The first and second radials are all 

 connected by a truncate side. The third radials are one-half wider 

 than high, hexagonal, contrary to the first and second, which are a 

 little higher. 



t They may have in some genera a representation in the vault, in which to some ex- 

 tent the plates of the calxy are repeated. Here the representatives of the basals and 

 radials ate frequently separated by one or more rings of intercalated pieces. Tli 

 vault pieces increase in number by age, and are often entirely absent in young specimens 

 or in small species of the genus, being here evidently a product of growth. In Acrocrinu* 

 one of the latest genera of the Palaeoerinoidea, the intercalated plates in the calyx may 

 have a similar origin, but are here evidently not mere individual growth, but have become 

 a fixed character. 



