92 On the Silurian System. 



posing on a stratum similar to that termed plastic clay, whilst this 

 latter is found at a short distance, towards the primary boundary. 

 I do not say that it may not occur beneath the fossiliferous beds 

 of the lower tertiary, although it has never been found in this rel- 

 ative position, but in the vicinity of Piscataway, Maryland, it re- 

 poses on the latter strata, and forms the hills around, whilst the 

 tertiary fossils are found in the beds of creeks and bottoms of ra- 

 vines. This clay is precisely similar to that around Baltimore, 

 but unfortunately I could not find the lignite stratum, which char- 

 acterizes the plastic clay in so many places, over a great extent of 

 its course throughout the Union. But as that is a thin stratum, 

 its absence must be expected in many localities of this formation. 

 The evidence then, so far, is in favor of the opinion that this clay 

 overlies the fossiliferous strata of the lower tertiary, and, there- 

 fore, does not exactly correspond in position to the plastic clay of 

 England which underlies the beds of marine shells of the eocene 

 period; still, they all belong to one era, and appear to have been 

 deposited in estuaries or in fresh water in the beds of rivers near 

 the sea. These remarks are made to call the attention of the 

 state geologists to this subject, and it is hoped the question wili 

 soon be determined. 



Ohservatio7is on the Genus Gnathodon, with description of a 



new species. 



Until recently, but a single species of this interesting genus 

 was known to naturalists, the G. cuneata, (Gray,) an inhabitant 

 of the estuaries of the Gulf of Mexico, and occurring in the up- 

 per tertiary formation in the bank of the Potomac river in Mary- 

 land, and on the Neuse river, North Carolina. The second species 

 described, was found in the high bank at Yorktown, Virginia, and 

 is only known as a fossil. The third, which I now describe, is a 

 recent species from Florida, which I owe to the kindness of Dr. 

 Forman of Baltimore. The three species are very distinct, and 

 the differences may be briefly stated in the following characters. 



1. G. cuneata. Anterior and posterior lateral teeth arched, 

 the latter being more than twice the length of the former. 



2. G. Grayi, (fossil.) Anterior tooth not greatly shorter than 

 the posterior ; both nearly straight. 



3. G.fiexuosa. Lateral teeth not greatly differing in length, 

 both much shorter than in the preceding species, and rectilinear. 

 It is a smaller species than the others. 



