INVERTEBRATES. 205 



rib passing up each of the rays, and their subdivision, out to the arm-bases, 

 and in the forms of some of the pieces. These characters give this crinoid a 

 very distinct aspect, but the structure being essentially the same, we are led, 

 since seeing several specimens showing various intermediate gradations in these 

 characters, to regard these forms as probably belonging to the same species. 



We had not seen Prof. Hall's description of A. rusticus * until after our diag- 

 nosis of A. Sillimani was printed, or we should certainly have observed its 

 exact agreement with the type of the latter. 



Locality and position : Upper part Burlington limestone, of Subcarbonifer- 

 ous series, Burlington, Iowa. 



Subgenus BATOCRINUS, Casseday. (See p. 150.) 

 Section d. 



ACTINOCRINUS DODECADACTYL'US, M. and W. 



PI. 15, fig. 3 a, 3 b, 3 c. 



Actinocrinus dodecadactylus, Meek and Worthen, June, 1861. Proceed. Acad. Nat. 

 Sci., Philad., p. 131. 



Body rather small, subglobose; summit and calyx below the 

 arms, of nearly the same size; breadth a little greater than the 

 height ; composed of slightly convex, smooth or subgranulose 

 plates, which are connected by moderately distinct sutures. 

 Base small, much depressed or subdiscoidal, obtusely hexa- 

 gonal in outline, with three other obtuse, retreating angles at 

 the sutures. First radial plates wider than long; three of them 

 regularly hexagonal, and two heptagonal. Second radial pieces 

 much smaller than the first, about twice as wide as long, and 

 all quadrangular. Third radials a little longer than the second, 

 all regularly hexagonal, the two inferior lateral margins being 

 very short, and the two superior sides each about equaling the 

 base. In the two posterior rays the third radial pieces each 



* We of course allude here to his description published in the Boston Journal Nat. 

 Hist., vol. vii, p. 269, and not to his very brief preliminary notice of three lines, pub- 

 lished in some sheets issued at Albany in Feb., 1861, in which the formula of the arms 

 was said to be, " as far as known, four to each ray." 



