Perhaps you have an older, obsolete version of the T&T "Handbook Of Civil War Bullets & Cartridges." In the Revised 2007 Edition, on page 25 it says T&T-65 has a cone cvity. At the bottom of that page there is a photo showing three bullets, with text saying:
".45-70 Sharps -- Previously believed to be Whitworths, these have since been identified as coming from post civil war Sharps metallic cartridges (1876-1880s). Dished base with [raised] dot in center. Long and short variants."
Regards,
Pete