NATURAL SCIENCES OF PHILADELPHIA. 395 



nearly flat plates, which are connected by moderately distinct sutures. Base 

 basin-shaped, truncate below, and expanding gradually upward, about half as 

 high as wide, and composed of nearly equal pentagonal pieces; columnar facet 

 large, concave, without a projecting marginal rim, pierced by a pentapetalous 

 central opening, less than one-third the diameter of the column. Subradial 

 plates unequal, some of them as long as wide, and others wider than long, 

 three hexagonal, and two of the larger ones on the anal side, heptagonal, 

 First radials rather smaller than the subradial pieces, all wider than high, 

 pentagonal, somewhat unequal, and horizontally truncate above, the upper 

 side being longer than either of the inferior sloping sides, which are longer 

 than the lateral margins. Second radial pieces very short, apparently sub- 

 quadrangular. Third radials short, or about three times as wide as high, pen- 

 tagonal, and supporting on their superior sloping sides the main divisions of 

 the arms. 



The first anal piece is pentagonal, and rests between the two upper sloping 

 sides of two subradials. On the right it supports one inferior sloping edge of 

 a first radial plate, and connects with another anal plate on the left, which 

 rests on the superior truncated end of one of the subradials. Both of these 

 anal pieces are truncated above, and surmounted by two others, the former of 

 which cannot be made out in the specimen described. 



The arms are very long, robust, rounded, and after the first division on the 

 third radial piece, bifurcate again on the eighth, ninth, and eleventh pieces, 

 in three of the arms examined, above which they seem to be simple, and are 

 each composed of a single series of pieces. The column is composed, near the 

 body, of firmly anchylosed pieces, the sutures of which are not visible 0-20 inch 

 below its connection with the base. Its central cavity presents, in a transverse 

 section, the same pentapetalous form as the perforation of the base. 



Length from base to summit of third radial plates, 0-79 inch ; breadrh 

 about 0-62 inch; breadth of base, 0-38 inch ; height of do. 0-19 inch ; length of 

 arms, about 350 inches. 



Named in honor of Prof. G. C. Swallow, State Geologist of Missouri. 



Locality and position. Burlington, Iowa. Burlington Limestone of the sub- 

 carboniferous series. 



ECHINOIDEA. 

 Genus AB.CHJEOCIDARIS, McCoy. 



Arch^ocidaris mccroxatus. The only specimens of this species yet obtained, 

 consist of detached primary spines, and a few of the interambulacral plates. 

 The plates are hexagonal, wider than long, and apparently nearly smooth, or 

 only ornamented by a single row of small tubercles around the margin. Cen- 

 tral tubercle prominent, and nearly equalling one-third the greater diameter 

 of the plate, most elevated in the middle, which has a small central pit, and 

 is separated from its surrounding lower margin by a distinct annular groove. 



Primary spines long, tapering, a little compressed, or nearly round, and 

 very slightly curved above the articulating extremity, near which they swell 

 out so as to form a distinct, smooth, somewhat angular ring ; from this to the 

 articulating end, they contract abruptly ; pointed at the upper extremity, and 

 armed by rather strong, sharp, but short lateral spinules, which are directed 

 obliquely outwards and upwards. Even under a magnifier the entire surface 

 generally appears to be smooth, but when a well preserved specimen is ex- 

 amined with a good lens, in a favorable light, it is seen to be marked by ex- 

 tremely fine, obscure, closely arranged, longitudinal strise. The articulating 

 end is distinctly perforated, and transverse sections show the central cavity 

 to be comparatively large for some distance above. 



Length of one of the interambulacral plates, - 50 inch; breadth of do. 0*35 

 inch. Length of primary spine, 2*62 inches ; greatest diameter of same above 



I860.] 



