I've been doing some research on S.teretidens since Nick suggested I should add it to this website. And I've got to say the reference material out there is a bit ambiguous. Just when I think I've got the hang of identifying them, I find an exception!
Here's a selection of the websites I've been hitting:
http://www.elasmo.com/frameMe.html?file=genera/cenozoic/sharks/sylvestrilamia.html&menu=bin/menu_genera-alt.html
http://users.skynet.be/belgiansharkteeth/pubwww/yperiaan/sharks/Sylvestrilamia%20teretidens.htm
http://phanerozoic.over-blog.com/article-sylvestrilamia-teretidens-de-l-ypresien-de-belgique-74337643.html
https://www.buriedtreasurefossils.com/shop-by-sharktooth-locality/belgium/carcharias-teretidens.html
https://www.mineralienatlas.de/lexikon/index.php/FossilData?fossil=Carcharias%20teretidens
http://www.trg.org/downloads/fossils%20of%20abbey%20wood.pdf
http://www.elasmo.com/frameMe.html?file=genera/cenozoic/sharks/striatolamia_new.html&menu=bin/menu_genera-alt.html
It's fairly clear to me that the most glaring difference between S.teretidens and S.macrota lies with the side cusplets. Mainly, with S.teretidens they are proportionately bigger and more robust than S.macrota. But if you look at page 4 of the pdf on the website 2nd from bottom in the above list - which gives examples of both side by side - this doesn't cover everything.
Another striking thing about the reference photos of S.teretidens is that, as Nick suggests, they look like small (juvenile) 3rd or 4th anteriors from S.macrota. In other words, a bit on the triangular side. Even ones that are a little more slender are noticeably wider at the base and more robust than, other S.macrota anteriors.
Anybody have a definitive guide out there?