In Whitehead & Russell's PM, what is the name of that symbol in series of segments? See the last line. It looks like an unfinished S.
Thanks!

 A: This is the final form of lowercase $\sigma$ (used when a word ends in $\sigma$), in LaTeX $\verb|\varsigma|=\varsigma$.
A: To expand on Yuval Filmus's answer: Yes it is the final form of sigma. To prove this, we also need to rule out the possibility that the symbol is stigma, a Greek ligature combining sigma and tau. The glyphs for stigma can vary drastically; sometimes it is quite similar to the final form of sigma, sometimes quite different. Take a look at how stigma is rendered in various modern fonts (the graphic shows uppercase and lowercase stigma, as well as uppercase and lowercase koppa/qoppa):

(Image taken from http://commons.wikimedia.org/wiki/File:Greek_Stigma_and_Koppa_font_design.svg.)
Due to the visual similarity with $\varsigma$, it's not easy to definitively decide whether the symbol is sigma or stigma based on appearance alone.
With these concerns in mind, I dug up some evidence which demonstrates that the symbol is in fact final-form sigma. The following is taken from a 1912 Greek text (the Loeb Classical Library 1912 edition of Philostratus, The Life of Apollonius of Tyana, The Epistles of Apollonius, and the Treatise of Eusebius, Volume 2, page 412):

The first letter in the picture is a stigma (in its role as the numeral 6); the last letter in the picture is a word-final sigma. The font here is identical to (or at least closely matches) the font in the text from Russell and Whitehead in the question: We can tell by the matching kappas and epsilons (visible in the picture in the question for comparison), as well as (not visible in that picture) by the curve of the lambdas, the tendency of the phis to have a tapering vertical line, and other similarities. And the word-final sigma matches the symbol in Russell and Whitehead, which was what we wanted to show. 
