134 PROCEEDINGS OF THE ACADEMY OP 



under examination is not in a condition to show whether or not there were any 

 other hifurcations. They all extend out nearly horizontally at first, and gradu- 

 ally curve upwards. Resting upon the inner sloping sides of each pair of 

 first brachial pieces, there is a rather large interaxillary piece, with three or 

 four much smaller pieces above. 



The vault is moderately convex, and composed of innumerable minute 

 pieces, and is provided with a central, or sub-central proboscis. It appears 

 to have continued out over the anus as far as to the third or fourth ranges of 

 small alternating arm pieces. 



This species evidently belongs to the same group as that on which M. 

 de Vernuil proposed to establish the genus Pradocrinus, which Prof. Koninck 

 thinks does not differ from Ctenocrinus of Bronn. 



Locality and position. Burlington, Iowa. Burlington Limestone. Collec- 

 tion of Mr. Charles Wachsmuth. 



Actinocrinus Sillimani. Body of moderate size, distinctly stelliform, or 

 pentalobate, as seen from below or above, in consequence of the deeply sinu- 

 ous character of the interradial and anal spaces ; rapidly spreading from the 

 base to the third radials, which, with the secondary and tertiary radials, and 

 brachial pieces, extend out horizontally. Summit nearly flat and provided 

 with a rather small sub-central proboscis. Basal pieces short, thickened and 

 projecting down over the summit of the column, so as to form a distinctly 

 trilobate rim, deeply indented at the sutures; columnar facet concave, and 

 rather less than one-third the breadth of the base. First radial plates about 

 as large as the anal pieces, very thick and prominent ; wider than high, three 

 of them heptagonal, (the angle at the middle of the under side being very 

 obtuse,) and two hexagonal. Second radial pieces smaller than the first, 

 thick and prominent, wider than high, and hexagonal in form. Third radials 

 about the size of the second, wider than long, heptagonal or hexagonal, and 

 supporting on each superior sloping side a secondary radial. Each of the lat- 

 ter supports on its outer side a series of brachial pieces, and on its inner side 

 tertiary radials, on the second of which another bifurcation takes place, making 

 six arms to each ray, or thirty in the entire series. The two. secondary, and 

 the four tertiary radials, as well as two or three of the brachial pieces on each 

 side of them, are covered in above by vault pieces. The first anal plate is 

 hexagonal, and nearly as large as the first radial pieces. In the next range 

 above, it supports two smaller hexagonal pieces, which in their turn support 

 three pieces in the next range, the middle one of which is larger and longer 

 than the others. On each side of this larger middle piece, there are two other 

 smaller pieces resting upon the two lateral pieces of the third range, and con- 

 necting with the vault above, and with the brachial pieces on each side. The 

 first anal plate is regularly hexagonal, and supports two smaller pieces in the 

 second range, over which there are three or four still smaller pieces connect- 

 ing with the vault above. 



A marked feature of this species is the prominence, and very profound 

 sculpturing of its plates. Its first radial and first anal pieces rise into elevated 

 transverse nodes, which occupy nearly their entire surface, and project beyond 

 the base, so as to present a distinct six-lobed outline, as seen from below. 

 From the under side of each of these prominences, one or two short ribs con- 

 nect with the base, while similar ribs connect them with each other on each 

 side. A single, rather prominent rib also passes from each to a smaller node 

 on each second radial piece above. 



The two second anal pieces, and all the first interradials are provided in the 

 middle with a rounded, rather small, very prominent subspiniferous node. 

 The third radial pieces have a central prominence, connecting by short carinae 

 with the node on the second radial below, as well as with each of the second- 

 ary radials above. On the latter pieces, these carinae bifurcate, and extend 

 along the tertiary radial and lateral brachial pieces, leaving deep depressions 



[June, 



