NATURAL SCIENCES OF PHILADELPHIA. 135 



between. The upper anal and interradial plates are convex, but do not gene- 

 rally rise into distinct nodes. 



Named in honor of the venerable Prof. B. Silliman, Sr., of New Haven, 

 Connecticut, who perhaps did more to create an interest in scientific studies 

 and pursuits, at an early period in the history of this country, than any other 

 person now living. 



Locality and position. Clear Creek, Warren County, Illinois. Burlington 

 Limestone. 



Genus AGARICOCRINUS, Troost. 



Agaricocrinus gracilis. Body small, truncato-subglobose exclusive of the 

 arms ; under side concave, the concavity extending out to the middle of the 

 second radial, first interradial, and second range of anal pieces, all of which 

 are geniculated, or abruptly bent upwards, and swollen, so as to form a circle 

 of hemispherical nodes around the margins of the concave under side. 



Anal and interradial pieces rising vertically from the margins of the con- 

 cavity below. Dome rather depressed, and provided with a short, strong, 

 sub-central spine. Anal opening small, not protuberant, and placed about on 

 a level with the arm openings, which are located slightly above the middle of 

 the entire body, including the dome. 



Base small, concave, nearly or quite hidden by the column. First radial 

 plates flat, included within the concavity of the under side, about as wide as 

 long, and apparently all hexagonal. Second radials slightly larger than the 

 first, and quadrangular in form. Third radial pieces shorter and wider than 

 the second, and each provided with an obtuse mesial ridge, which connects 

 with the prominence of the second radials below ; sub-heptagonal in outline, 

 the superior angle being rather salient, and the slopes on each side supporting 

 the first brachial pieces. First interradial pieces larger than the first or 

 second radial plates, wider above the geniculation than below ; irregu- 

 larly octagonal in form, and supporting two smaller elongated pieces in 

 the next range above. First anal plate small, and included within the con- 

 cavity of the under side ; supporting in the next range three larger pieces, the 

 middle one of which is smaller than the other two. Above these five smaller 

 pieces are seen in the next range, which is as far as they can be counted in 

 the specimen examined. After the first division on the third radial pieces, 

 the arms are each at first composed of a single nodose brachial piece, but 

 almost immediately pass into a double series of short alternating pieces, which 

 support a closely set range of tentacles on each side within. As they do not 

 bifurcate again, there are but two arms to each ray, or ten in the entire series. 

 Near the base the column is rather slender, round, and composed of alternately 

 thicker and thinner plates, with a very small round central perforation. The 

 surface is finely and regularly granulose where well preserved, but generally 

 appears to be smooth. 



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

 tion of Mr. Charles Wachsmuth. 



Platycrinus Mui/n-BRAcniATus. Body below the summit of the first radial 

 pieces depressed, basin-shaped. Base discoid, pentagonal in outline, moder- 

 ately concave below, and not prominent enough to be seen in a side view ; 

 columnar facet about one-third as wide as the base. First radial plates large, 

 broader than high, and widening rather rapidly upwards from the base ; facet 

 for the reception of the second radial pieces prominent, extending down appa- 

 rently below the middle of the plate. Second radial pieces small, trian- 

 gular, wider than long, and extending obliquely outwards and upwards from 

 the first radials. 



After dividing on the second radials, the arms bifurcate again on the second 

 piece, and the two inner divisions, which are larger than the others, bifurcate 

 again on the second piece, above which two of the sub-divisions in some of the 



1861.] 



