NATURAL SCIENCES OF PHILADELPHIA. 147 



This species will be readily distinguished from P. conicus, of Billings, by 

 its broader, more ovoid, and more angular form, owing to the much greater 

 prominence of its subradial pieces, and particularly by the well-defined ridges 

 radiating from the centre of the plates. In the latter character, it approaches 

 more nearly the last described species, P. crassus, from which it differs in a 

 marked degree, in having its under side below the middle of the subradial 

 pieces greatly more tapering, and base much smaller, and not wider than the 

 head of the column, as well as proportionally higher. It also differs in having 

 its greatest breadth at the middle of the subradial pieces, which are much 

 more prominent ; while its pectinated openings are not sunken, nor its sutures 

 furrowed as in the last. 



Locality and position. Trenton Limestone, of Lower Silurian; Dixon, 111. 



Genus HETEROCRINE, Hall, 1847. 

 Heterocrinus crassus, M. & W. 



Body robust, but rather small compared with the arms and column, wider 

 above than the length from the base to the summit of the first radials ; dis- 

 tinctly truncated at its connection with the column, from which point the 

 sides expand rather distinctly upwards ; subpentagonal in outline as seen 

 from below. Basal pieces pentagonal, wider than long, and all excavated or 

 indented on the outside at the superior angle and down the middle. First 

 radial pieces longer than the basal, about three-fourths as long as wide, 

 broadly truncated above, and regularly pentagonal in form, excepting two on 

 the anal side, which appear to each have one of the superior lateral angles a 

 little truncated for the reception of a small anal piece ; all deeply indented 

 at their inferior lateral angles, so as to leave a broad, rounded, undefined ridge 

 or prominence descending from the middle to the basal pieces. Succeeding 

 radial pieces forming free arms, nearly as wide as, but much shorter than, 

 the first; in four of the rays all transversely oblong, and about three or four 

 times as wide as long, excepting the fourth or fifth pieces, which is pentagonal, 

 and supports, on its sloping upper side, the first divisions. In one ray on the 

 anal side the second piece is pentagonal, larger than that of any of the others, 

 and gives off a lateral branch from its short sloping side on the left,* above 

 which the other pieces present the same size and form seen in the other rays. 



Arms after the first division on last radial, bifurcating again on the sixth or 

 seventh piece, after which they are known to divide again in one arm, on the 

 sixth piece, which is as far as our specimens show the structure. 



Surface usually appearing smooth, but sometimes showing traces of scatter- 

 ing granules. Column comparatively large, distinctly pentagonal, and ex- 

 panding upwards near the base of the body, where it is composed of irregu- 

 larly alternating thicker and thinner segments ; central perforation small and 

 round. 



Height of body from base to the summit of first radial pieces, 0*35 inch ; 

 breadth at summit of first radials, about 0-67 inch ; length of five succeeding 

 radial pieces, - 4(j inch ; breadth of do. about 0*15 inch. Breadth of column 

 at its connection with the base, 03:2 inch. 



This is perhaps the largest and most robust species of the genus known. It 

 is composed of thick, strong plates, and the indentations or excavations at 

 the points where the superior angle of each basal plate connects with the 

 inferior lateral angles of the first radials, together with the more shallow 

 depressions extending down from these points to the lower margin of the basal 

 pieces, give a pentagonal outline to the body the five angles being coincident 

 with those of the column. 



* From some of the specimens it appears somewhat doubtful whether this may not be. a range 

 of anal pieces, instead of a branch of the arm. 



1865.] 



