208 PALAEONTOLOGY OF ILLINOIS. 



The first anal plate is a little smaller than the first radials, 

 heptagonal in form, and supports three smaller pieces in the 

 next range, two of which are hexagonal and one heptagonal ; 

 above these there are two other still smaller pieces, and over 

 the latter, in the fourth range, three large arcuate plates, con- 

 necting with the vault above. The first iiiterradial pieces are 

 of moderate size, as long as wide, hexagonal, and each support- 

 ing two smaller pieces in the next range; above the latter there 

 are two or three larger, alongate pieces in the third range, 

 which arch inwards and connect with the summit. 



The vault is depressed, composed of numerous medium sized 

 convex pieces, and provided with a subcentral proboscis. The 

 arms are all broken from our specimens, but judging from the 

 size of the openings, they would appear to be very slender. 

 They seem to have ascended vertically from the upper side of 

 the second brachial pieces. Height to the base of the proboscis, 

 about 0.50 inch; ditto to arm openings, 0.35 inch; breadth 

 across from side to side, between the interradials, 0.67 inch; 

 greatest breadth across between the arm openings on opposite 

 sides, 0.93 inch. 



This form is thought to be a variety of A. sequibrachiatus of McChesney, in 

 which opinion we are inclined to concur, since seeing Prof. McChesney's figures, 

 recently published. As we have not yet seen specimens showing all the inter- 

 mediate gradations, however, connecting these two forms, although we have 

 several of each before us, we are not fully satisfied that we should consider 

 them identical. They are certainly very similar, there being no difference of 

 specific importance in the structure or arrangement of the parts composing the 

 body. The differences that must distinguish them, either as species or varie- 

 ties, are the more depressed and distinctly lobed form of the asteriscus, the 

 interradial and anal spaces being excavated nearly half way in from the arm 

 bases towards the center, so as to give the body a very distinctly five-lobed out- 

 line, instead of having merely slightly sinuous margins, as seen from above or 

 below. It will also be seen, from our figures, that even the axillary and i.nter- 

 brachial spaces in our crinoid are also somewhat sinuous, while the lobes formed 

 by its arm bases are more flattened, and its base more rounded, than in the 

 sequibrachiatus. At the same time that we are prepared to believe our form 

 may prove to be only a variety of A. sequibrachiatus, it is certainly more distinct 



