"and faced with its shell fanlight the changing Thames"

 

Fanlight
Creative Commons AttributionFanlight - Credit: Amanda Slater, Flickr
A fanlight is a window (usually semi-circular or semi-elliptical) above a door.

The term shell fanlight does not appear in the Oxford English Dictionary. However, it  may be used to refer to a fanlight where the area in which one normally finds glass is filled in with plaster moulded in the form of a shell.

Click here to see an example of a shell fanlight in London.