196 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



Distribution. Ordovicic and Siluric of North America. Con- 

 tains the following species: 



S. stellata (Billings). Trenton, Ottawa, Canada. 



8. laxata, new species. Richmond of Ohio. 



S. ordinaria, new species. Basal Siluric.* Girardeau lime- 

 stone, Alexander County, Illinois. 



Remarks. Schuchertia need only be compared with Petraster and 

 Palasterina. The former differs in that the incomplete columns of 

 inframarginals adjoin the adambulacrals and are not separated from 

 them by interbrachial accessory plates as in the other two genera. 

 Petraster is readily distinguished by the prominent columns of infra- 

 marginals bounding the entire animal and abactinally by the distinct 

 columns of radial, supramarginal, and ambital plates. In Schuchertia 

 the plates of the abactinal and interbrachial areas are alike and 

 not distinguishable into the kinds of columns just mentioned for 

 Petraster. 



The fact that in ScJiuchertia the " axillary marginals 7 ' and infra- 

 marginals remain beside the adambulacrals seems to indicate its 

 origin in forms like Hudsonaster. The generic structure of the latter 

 is retained in Schuchertia, to which is added along the margin (mainly 

 axillary) a series of interbrachial accessory plates. The generic 

 structure of Hudsonaster is also retained in Petraster, but here, unlike 

 those in Schuchertia, the interbrachial accessory plates are developed 

 between the inframarginals and adambulacrals, forcing these columns 

 apart. The phylum starting in Schuchertia is not a prolific one, but 

 the other, Palasterinidse, whose inframarginals are true marginals, 

 is not only more prolific in genera but also has a longer geological 

 range. Beginning in Petraster of the Ordovicic, it is continued in 

 the Siluric in Lindstromaster and Palasterina. 



SCHUCHERTIA STELLATA (Billings). 

 Plate 32, fig. 2; plate 33, fig.l. 



Palasterina stellata BILLINGS, Geol. Surv. Canada, Rep. Progress for 1853-1856, 

 1857, p. 290; Geol. Surv. Canada, Can. Org. Rem., dec. 3, 1858, p. 76, pi. 9, 

 figs, la, 16. WRIGHT, Mon. British Foss. Echinod., Oolitic, vol. 2, pt. 1, 

 (Pakeontogr. Soc. for 1861), 1862, p. 27, fig. 16 on p. 26. QUENSTEDT, Petre- 

 factenkunde Deutschlands, vol. 4, 1876, p. 74, pi. 92, fig. 34. 



Trentonaster stellata STURTZ, Verb, naturh. Ver. preuss. Rheinl., etc., vol. 56, 

 1900, pp. 217, 224, 225. 



Description of 1858. "Pentagonal; disk about one-half of the whole 

 diameter; ambulacral grooves narrow and deep, bordered on each 

 side by a row of small, nearly square adambulacral plates; a second 

 row consisting of disk plates extends nearly to the end of each ray, 

 the remainder of the disk covered with smaller plates. All of these 

 plates are solid and closely fitted together; the disk plates in the 



