Built in 1907 by plans of Albert Groves, the Mark Twain used to be the Maryland and Baltimore hotels. It almost certainly has lost its original cornice.
It contains some of the most sumptuous terracotta, including some now outdated swastikas.
The bay windows, while present in other downtown buildings, are particularly noteworthy here.
Story about the Mark Twain in the RFT from 2014: http://blogs.riverfronttimes.com/dailyrft/2014/08/mark_twain_hotel_downtown_changes_st_louis.php
From the P-D in 2010: http://www.stltoday.com/news/local/metro/problems-at-transient-hotel-downtown-draw-official-attention/article_a876fb5d-7af2-5b52-9d81-c701dbfdb1eb.html
Both give some info about its history.
I’m a big fan of Albert B. Groves. He designed the Monogram Building (1706 Washington – CPI Corp HQ) and several other terra cotta covered beauties that I grew to really appreciate while working at CPI. I’ve randomly stumbled upon cornerstones bearing his name all around the city, but the Mark Twain Hotel is definitely a standout.