On the Silurian System. 91 



WENLOCK SHALE. 



Shells. Trilohite. 



Strophomena transversalis, Asaphus limulurus, 



(Leptena, Sow.) (Wetherilli, Green,) 



Orthis elegantula, (longicaudatus, Murch.) 



(canalis, Sow.) 



hybrida, 

 Delthyris lineatus. 



CARADOC SANDSTONE. 



Shells. Trilobites. 



Strophomena alternata, Cryptolithus lessellatus,* 



sericea, (Trinucleus caractaci, Murch.) 



(Leptsena, Sow.) Calymene micropleura, 



Orthis testudinaria, (Asaphus micropleurus, Green,) 



callactis, (Calymene punctata, Murch.) 

 flabellulum, 

 Bellerophon acutus. 



Observations on the Plastic Clay. 



There appears to be some difference of opinion respecting the 

 relative position of this formation in the United States ; by some 

 geologists it is included in the secondary or cretaceous series, and 

 by others in the lower tertiary, corresponding in position to the 

 plastic clay of England. I have examined specimens from both 

 countries, and the only difference I can perceive, is the more 

 unctuous or glossy appearance of the English specimens. If of 

 the same age, it is remarkable that a tertiary deposit should extend 

 such a distance, and preserve its mineral character and its variega- 

 ted appearance so perfectly as it does, or rather that the condi- 

 tions for its deposition were so much the same in both countries 

 in the same era. But this resemblance will not alone establish 

 the relationship of the American with the plastic clay of Europe, 

 and fossils we have none, whereby to institute a comparison. In 

 Maryland and Virginia the lower fossiliferous tertiary rests either 

 on the primary or cretaceous rocks, and has never been found re- 



* Mr. Murchison thinks his species different from the tesseilatus, in consequence 

 of having long spinous processes to the buckler; but in this respect there is no 

 difference between specimens in Wales and at Glenn's Falls, in New York, where 

 I found them abundantly. The original specimens examined by ray friend Dr. 

 Green, were very imperfect, which accounts for the incomplete description of that 

 accurate observer. I cannot omit to pay a passing tribute of praise to the zeal, in- 

 dustry and talent, which have put us in possession of a monograph of the trilobites, 

 illustrated by models, that have greatly facilitated the labors of geologists, and 

 stimulated inquiry into the history of Transition remains. 



