REVISION OF PALEOZOIC STELLEROIDEA. 199 



Ohio. Another fragment was found by Mr. E. H. Vaupel at Oxford 

 in the same State. 



Remarks. The only Upper Ordovicic species with which Schu- 

 cTiertia laxata can 136 confounded is Petraster speciosus. However, 

 there is one fundamental difference between them, so that even 

 fragments can be distinguished. In P. speciosus the inframarginals 

 are very conspicuous and bound the outer margin of the entire animal, 

 while in S. laxata there are no columns of marginal plates, since 

 the inframarginals closely adjoin the adambulacrals. In other 

 words, in the former species the interbrachial accessory plates are 

 between the adambulacrals and inframarginals, while in the latter 

 form these pieces, which are also far more numerous, are all outside 

 of the inframarginals. The generally disjointed condition of the plates 

 in S. laxata will probably always distinguish it from P. speciosus. 



Cat. Nos. 60613, 60623, U.S.N.M. 



SCHUCHERTIA ORDINARIA, new species. 



A regular species of Schuchertia. 



The holotjTpe shows the abactinal side only. R = 1 1 mm., r = about 

 6.3 mm. 



This species is most closely related to S. laxata, but is smaller and 

 with a comparatively larger disk. Therefore the rays do not protrude 

 beyond the disk so far as in the latter form. Abactinal skeleton con- 

 sists of minute tumid ossicles that are more cut along their edges 

 than in S. laxata, and therefore more spicular. 



The specimen can not be freed of the adhering clay sufficiently to 

 make an instructive photograph, and is therefore not illustrated. 



Formation and locality. The holotype is from the basal Siluric 

 Girardeau limestone, Alexander County, Illinois. It is in the Gurley 

 collection of the University of Chicago (No. 10992). 



Family PALASTEBISCID^E Gregory. 



Palssbrisingidx STURTZ, Palaeontographica, vol. 36, 1890, p. 246. 

 Palsechinasteridse STURTZ, Palaeontographica, vol. 36, 1890, p. 246, also p. 247. 

 Palasteriscidse GREGORY, Treat. Zool., vol. 3, Echinoderma, 1900, p. 257. 



Specialized Cryptozonia with very large interbrachial arcs. Abacti- 

 nal and interbrachial areas either with a smooth membrane or granu- 

 lar and thorny. No radial or supramarginal columns of ossicles. 

 Inframarginals not present in any position. Madreporite probably 

 always actinal. Ambulacrals alternating or opposite; adambulacrals 

 more or less modified, but remain adjoining the ambulacrals. 

 Contains : 



Palasteriscus Sturtz. 

 Echinasterella Stiirtz. 

 Loriolaster Sturtz. 

 Cheiropteraster Sturtz. 

 For remarks on the course of development, see Schuchertiidse. 



