The Original Clayface (Basil Karlo)

First appearance: Detective Comics #40 (cover-dated June 1940),
though he was seen in the last panel of the last page of the Batman (and Robin) story in Detective Comics #39:



NOTE: There has been the possibility raised that the person with the hatchet on the cover is NOT Basil Karlo since Batman and Robin were dealing with a killer who used a hatchet in the previous issue of Detective Comics.
Also from Detective Comics #39:


Now, back to our story:



And, in the famous words of the serials of yesteryear, "To be continued . . . "