294 PALAEONTOLOGY OF ILLINOIS. 



ECHINOIDEA. 



PERISCHOECHINID^E. (See page 225.) 



Genus ARCH^EOCIDARIS, McCoy. 



Synon. — Echinocrinus* Agassiz, 1841. Monogr. Ech., ii, Introd., p. 15; McCoy, 1844, 

 Carb. Foss. Ireland, p. 173. 

 Archseocidaris , McCoy, 1844. lb. 

 Palseocidaris, Desoe, 1846. Cat. Rais., p. 58. 



Spherical or depressed spheroidal. Ambulacra compara- 

 tively narrow and composed of two ranges of small pieces, 

 with two pores to each piece. Inter-ambulacral plates large, 

 thin, and each provided with a large, perforated, central tuber- 

 cle, surrounded at its base by a smooth ring, and rounded at the 

 extremity for the articulation of the primary spines ; surround- 

 ing the whole is a more or less complete circle of marginal, or 

 submarginal, smaller tubercles, for the articulation of the sec- 

 ondary spines. Apical disc unknown. Mouth surrounded ap- 

 parently by a membrane, covered by numerous minute, imbri- 

 cating plates. Jaws strong, trigonal, with a distinct mesial 

 suture. Primary spines large, cylindrical, compressed, or 

 angular, and generally armed with small, oblique, lateral 

 spine-like projections; surface variously ornamented with gran- 

 ules and fine striae. 



This genus is nearly allied to Eocidaris, of Desor (Synop. Ech., 1858, p. 155), 

 from which, according to him, it is mainly distinguished by the presence of 

 the ring around the base of the* primary tubercle of each interambulacral plate. 

 From Perischodomus, of McCoy, it is readily distinguished by having a large 

 central tubercle in each of its interambulacral plates, instead of only on those 

 of the marginal rows, on each side of the ambulacra. 



Prof. Hall has described a very remarkable type from the Burlington group, 

 under the name Lepidechinus (Descr. New Sp. Crinoidea, Prelim. Notice, 



* The name Echinocrinus, which has priority of date for this genus, has probably 

 not been adopted by Palaeontologists, because they think it liable to mislead the stu- 

 dent in regard to the affinities of these fossils. 



