650 On the Vertical Range of Certain Fossil Species  [July, 
This statement may be strictly true, but it may be urged, on 
the other side, that there is no distinct character by which O. 
tulliensis can be separated from O. impressa. No doubt two forms 
can be picked out as types of the so-called species, but their vari- 
ation is considerable and they run into one another, It would be 
exceedingly difficult, perhaps quite impossible, to separate accu- 
rately a mixture of the two species if the horizons of the speci- 
mens were unknown. We seem here to have an example of a 
vicious reasoning of which palzontology affords not a few simi- 
lar instances. The Orthis from the Tully limestone is called O. 
tulliensis ; that from the overlying Chemung is called O. impressa. 
But barring this difference it would be hard to draw a line be- 
tween them. The practice of giving different names to fossils 
simply because they come from ‘different horizons has been car- 
ried too far. It has created artificial barriers in the way of tracing 
the evolution of species and the connection of strata. Neither 
geological nor geographical separation is sufficient reason for 
distinguishing by name fossils between which no clear structural 
difference can be pointed out. The cases are few, especially 
among the Testacea, that justify resort to the argument used by 
the late Professor Meek, “that if the whole structure of the ani- 
mal had been preserved doubtless some distinctions would be 
found which do not exist in the fossil.” 
Referring to the extract given above from the preface of Gy, 
Professor Williams says in the letter already quoted: 
“In regard to Orthis tulliensis it may be said that the common 
Orthis occurring at the base of the Ithaca fauna within a few 
hundred feet above the Genessee shale (less than 500) resembles 
at its first appearance Orthis tulliensis in form and general charac- 
ter, though for distinctness it may be appropriate to call it a vari- 
ety of Orthis impressa, since a little higher and in the same fauna 
the typical Orthis impressa appears in abundance. 
“Still there are specimens in the collection, from the lowest 
zone, which it would be difficult for any one to distinguish by 
microscopic or macroscopic characters from O. tulliensis.” 
There is therefore in New York an Orthis which cannot be dis- 
tinguished from O. tudliensis, occurring not at 200 feet only, but 
at a yet greater height (less than 500 feet) above the Genessee 
shale. | 
in regard to the association of this fossil with S. mesocostalis, 
2 Professor Williams adds 
~ “T have no single slab containing this form with S. mesocostalis, 
