41 



it is reduced to less than one-third its greatest width, whence it re- 

 mains stationary as far as it is preserved in the specimen. 

 The segments of the upper or conical part are short, sharply-edged, 

 and of the same height, but as soon as the column attains its ordi- 

 nary size, the joints become abruptly higher, more cylindrical, and 

 thicker and thinner joints alternate in the usual way. Central per- 

 forjition small. 



The specific name is given in honor of P. Herbert Carpenter, 

 Assistant Master at Eaton college, England, one of the founders of 

 the genus Allagecrinus* 



Geological jJosition, etc.: From the Chester or Kaskaskia lime- 

 stone, Monroe county. 111. 



Illinois State collection. 



ACROCEINUS WORTHENI. N. SP, 



This species is described from a single specimen, of which only 

 the calyx is preserved, the vault, the arms, and the column are so far 

 unknown. The calyx, however, is in excellent preservation, and, 

 notwithstanding its small size, exposes plainly every plate. The 

 form is calyculate, broadly truncate at the bottom, abruptly bending 

 upward toward the base of the first radials, whence it gradually de- 

 creases in width to the arms. Its length is 40.100 of an inch, its 

 greatest width 42.100, the width at the arm-bases 34.100. The plates 

 are plain, without ornamentation, but sufficiently convex to point 

 out the sutures. 



The genus 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 inter-radial, and which have apparently no rep- 

 resentation in other genera of the Palaeocrinoideat 



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 pos- 

 terior 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 



* The genus was proposed by P. Herb. Carpenter and Dr. E. Etheridge. jun., for a 

 small species from the Carboniferous of Scotland. [Annals and Magazine Nat. hist., 

 Aprill881, p. 281. 



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

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

 radials are frequently separated by one or more rings of intercalated pieces. These 

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

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

 Acrocrinus, one of the latest genera of the Palieocrinoidea, 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. 



—6 



