On the west and north elevations of the building, the fourteen keystone heads were being sculpted by Catherine Mawer's husband Robert, between 1853 and 1854, when he died. Catherine Mawer completed the masks, as well as the putti on the side panels of the main entrance and on the clock tower. The four Portland stone lions on plinths along the frontage, an 1867 addition by the sculptor William Day Keyworth Jr, contrast with the sandstone of the building itself, and were modelled at London Zoo. The first two lions were unveiled on 15 February 1867, the soft Portland stone has subsequently eroded with the weather.