wili.iams] SPECIES OF THE DELTHYKIS SHALY LIMESTONE. 73 
latter half of the growth larger in size, thus making a narrow form 
with sharper beak. This species is R. nobilis. 
b. The second dominant group of the Upper Pentamerus is a fur- 
ther mutation of R. nucleolata of the preceding stage; the plications 
are still fewer, and the gibbosity is still more extreme. This is JR. 
ventricosa. 
6. In the Oriskany the elongate-oval form is an expression of the 
same tendency seen in R. nobilis, but it is associated with smaller num- 
ber of ribs and less dominant fold and sinus and much greater size 
than in the Lower Helderberg forms. This is R. speciosa. 
A study of the plications on the forms from the several Lower 
Helderberg localities of New York shows the intimate relationship 
between the evolution of these characters and the ages, or, more 
strictly, successive stages in geologic history, at which they occur. 
Without going outside of the published report of the facts as found 
in Vol. Ill of the Paleontology of New York, the following points 
are in evidence: 
A. R. mutabilis is reported as having twenty to twenty-six plica- 
tions in all, and six to eight on the fold. This is the Lower Pentame- 
rus species. 
B. R. pyramidata, thirteen to twenty-two, and four to six on fold. 
R. nucleolata, fifteen to twenty-three, and four to five on fold. 
R. abrwpta, twenty-five to thirty-three, and seven to eight on fold. 
R. vellicata, twenty-four to thirty -six, and six to eight on fold. 
These are the Delthyris Shaly limestone species. It will be observed 
that R. mutabilis is strictly intermediate, and the first two species are 
different by having less and the second two by having more plications 
than the earlier species. 
C. The next series are from the following stage, the Upper Pen- 
tamerus: 
R. ventricosa, fourteen to twenty, and three to four on fold. 
R. campbellana, twenty-two to twenty-four, and five to six on 
fold. 
R. nobilis, twenty -six to thirty-two, and six to eight on fold. 
This set exceeds the extremes of the set of the preceding stage in 
some particulars, but the limit of variation in each case is less. This 
latter characteristic, which may be described as a tendency to restric- 
tion of variation, is to be observed in a collection of specimens by the 
fact that the specimens in a collection of R. ventricosa are more uni- 
form than those in a similar collection of the species R. pyramidata; 
or a set of specimens of R. nobilis may be said to* be more uniform 
than a set of specimens of R. vellicata. 
D. When we pass up to the Oriskany the plications are conspicu- 
ously increased, with decided increase in the size of the specimens. 
Again taking the reported statistics, supplementing them with a 
