330 PALEONTOLOGY OF ILLINOIS. 



ORDER OPHIUEOIDEA. 



TREMATASTEB, n. gen. 



(Ety. trema, an opening; aster, a star.) 



Body stellate, central part discoid, rays long, flexuous and gradu- 

 ally tapering. Kays consisting of a double series of ambulacral 

 plates, forming, together, a subcuneiform series with the tapering 

 ends directed toward the apices of the rays and uniting with the 

 larger ends of the succeeding double plates, upon each side of which 

 there is a series of curved adambulacral plates, which form the 

 margins of the rays. The pores are large and situated between the 

 contracting sides of the ambulacral plates and the concave sides of 

 the curving adambulacrals. Four plates border upon each pore 

 though nearly surrounded by two of them. Plates ten. Dorsal sur- 

 face unknown. 



This genus is distinguished by the parallel arrangement of the 

 ambulacrals and adambulacrals, instead of the alternate order, and 

 by the large pores. 



TREMATASTEB DIFFICILIS, Sp. nov. 



PI. XXXI, fig. 3. a, ventral side magnified two diameters, fig. 3, b, section of a ray showing 

 the arrangement of the plates magnified four diameters. 



Body stellate, disc small, rays long, flexuous and gradually tapering. 



Ambulacral plates subtrigonal, elongated, and united upon their 

 straight faces in parallel order, so as to form a series of sub-* 

 cuneiform sections at the bottom of the ambulacral groove, with the 

 tapering end of each directed toward the apex of the ray, and uniting 

 with the larger end of the next succeeding double plate. 



Adambulacral plates parallel with the ambulacrals, and each forms 

 a curve from the larger end of an ambulacral toward the apex of 

 the ray, to unite with the next succeeding adambulacral as it curves , 

 away from its attachment to the ambulacral. By this order of con- 

 struction the adambulacrals are fixed, at one end, to the ambula- 

 crals, while the other end moves upon the curved surface of an 

 adambulacral so as to allow the same lateral flexibility of the rays, 

 secured in other genera by the alternate arrangement of the plates. 



The pores are large and situated between the contracting sides of 



