288 
The American Geologist. 
April, 1894 
given, is <it' exaggerated size) is the temporal precursor of the cometary 
outburst of Leptodesma in the Chemung faunas, and is one of the two 
species occurring in the Hamilton faunas beneath. 
[ am disposed to believe that whenever this method is practicable 
ii will be found of real usefulness nol only to biologists in general, but lu 
the palfeontologisl in particular, especially in the determination of spe 
cific standards or fundamenfa, as suggested by Uather. Its successful 
application implies, first of all, abundant data. With that prerequi- 
site provided for, profitable comparisons could be instituted between 
species of distant faunas, supposed to be identical, or which, though 
perhaps identical, maybe masquerading under differenl names; and 
between such species in successive faunas of the same region. Asa 
rase in point, thr brachiopod species Athyru or Seminula subtilita Hall 
would serve: an abundant and remarkably variable shell from the up- 
per Carboniferous. in the lower Carboniferous limestones occurs an 
allied form, A. nubquadrafa, and ii has been confidently asserted that 
neither species passes into the jreological horizon of the other. Vet the 
1 wo arc greatly alike and each appears under many variat ions of mil line 
and contour. Assuming that the two are alike in internal structure, 
which I believe lu be essentially true, a series of composifes might de- 
termine whether or not the variations of the 1 wo so-called species reduce 
io similar fundamenta, and if ihe\ do I should be inclined to regard this 
as rational evidence of specific identity. Again, the geological value of 
this method mighl be tested in such a case as that recentlj adduced l)\ 
Prof. Prosser, who argues that an extensive series of rocks in central 
