206 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



is inevitable that at all the localities the species comes from the 

 shales now known as Knobstone. This adds another strong confir- 

 mation to the correlation of these beds with the Lower Burlington; 

 for the stratigraphic succession of the blastoids is quite peculiar. 

 Certain groups of genera, to which the present one belongs, coming 

 up from Nucleocrinus and Codaster of the Devonian, became promi- 

 nent, culminated, and were practically extinguished in the Burling- 

 ton. Another line, beghining with Troosticrinus in the Silurian, con- 

 tinued along moderately until the St. Louis and Kaskaskia, when it 

 culminated in a development of the genus Pentremitcs in extraor- 

 dinary profusion. In the Keokuk, between these two culminations, 

 the blastoids were relatively rare. In the most prolific localities for 

 crinoids, such as Crawfordsville, Indian Creek, and in Washington 

 and Lawrence counties, Indiana, and in Barren and Metcalfe coun- 

 ties, Kentucky, specimens are scarcely ever found. The abundant 

 occurrences of Pentremites conoideus at Spergen Hill, Indiana, and 

 Boonville, Missouri, are from liigher beds above the Keokuk — namely, 

 the Warsaw. In the true Keokuk of the Mississippi River and Ken- 

 tucky region a few straggling specimens of Metahlastus, and the dimin- 

 utive Granatocrinus already mentioned, are all that are ever found. 

 It would seem, therefore, a needless stretch of the imagination to 

 assign such a species as this to a horizon in which its group is prac- 

 tically extinct, instead of to one where upon every consideration of 

 faunal succession and association it properly belongs. 



S. decussatus (Shumard).^ This species, described from Button- 

 mould Knob under Pentremites, is thought to belong to the above 

 genus. All the specimens are imperfect, and its exact generic char- 

 acters obscure, but it belongs to this group, and not to Pentremites as 

 now restricted. I have some other specimens from the type locality 

 and also from the Knobstone shales at Bradfordsville, Marion 

 County, Kentucky. It occurs also at Fern Glen, Missouri, in shales 

 directly underlying the typical Burlmgton Limestone, and is figured 

 as a part of that fauna by Weller.^ 



To show the full force of the foregoing facts, I have arranged in 

 tabular form the genera, and a few decisive species which are found 

 at the Knob locahties and Whites Creek, along with their known 

 stratigraphical hmits elsewhere. 



1 Pentremites decussatus, Trans. St. Louis Acad. Sci., vol. 1 p. 242. 



2 Bull. Geol. Soc. Amer., vol. 20, p. 288, pi. 11, figs. 28, 29. 



