336 PALEONTOLOGY OF ILLINOIS. 



ECHINODISCUS OPTATUS, Sp. nov. 



PI. XXXI, Fig. 9, view of part of the lower side of a specimen showing the plates near 

 the margin. 



Body discoid, rather large, being more than an inch in diameter, 

 and having large plates in the interambulacral areas, and smaller 

 ones on other parts of the test. The margin is composed of small 

 plates elongated in the direction of the circumference, and forming 

 a semi-cylindrical or rounded rim, but not attaching to other objects, 

 showing that if the species was sessile it did not unite by the whole 

 underside, as in Agelacrlnus. 



Interambulacral areas 'depressed, concave, covered with polygonal 

 plates, those in the central part large, while those in the margin 

 are smaller, and appear as if trimmed to a straight edge for the 

 reception of the ambulacra. 



The ambulacra are long, slender, curving, angular arches, set 

 within the face of the disc, and joined with the interambulacral 

 plates by straight lines, without interlocking or imbricating plates. 

 They are composed on each side of alternately longer and shorter 

 plates which interlock at the angular arch, with each other and 

 with smaller intercalated plates, so as to give a zigzag outline to 

 the dovetailing union of the plates on the surface of each ambu- 

 lacrum. The ambulacra do not arise, as in Agelacrinus, from a 

 central pyramid, but the divisions take place near the central part of 

 the disc, separated by numerous ambulacral plates, and on different 

 sides of the mouth. The manner of bifurcation may be described 

 in this way : a division takes place on one side of the mouth, from 

 whence two ambulacra curve over the face of the disc, and an 

 ambulacral series of plates is directed past the mouth; at the dis- 

 tance of one-tenth of an inch it throws off, in the direction oppo- 

 site the mouth, a curving ambulacrum and continuing to pass the 

 mouth, at the distance of another tenth of an inch it divides into 

 two ambulacra. This leaves the mouth in the wider interambu- 

 lacral area. 



The mouth would seem, from the condition of our specimens, to 

 be central, but it may be slightly subcentral. It consists, on the 

 exterior, of an elongated elevation covered by numerous plates of 

 larger and smaller size, which are supported on the interior by a 

 fewer number of plates, some of which are fluted, but the exact 

 order, arrangement and purposes of which are not determined. 



