The Heart of the Matter

Portrait of Doge Francesco Erizzo by Bernardo Strozzi, Gallerie dell' Accademia, Venice
Apart from a single exception, only the Doges of Venice have ever been accorded the privilege of being buried in the Basilica San Marco. And even then none was buried in the actual body of the church, only in the narthex (atrium) or in the baptistery. 
Plaque marking the spot where the heart of Doge Francesco Erizzo is interred, Basilica San Marco, Venice
The practise came to an end centuries ago; the last Doge to be so interred (in the baptistery) was Andrea Dandolo, who died in 1354. However, almost three centuries later a slight bending of the rules seems to have been made for Doge Francesco Erizzo (r. 1631-46). 

Embedded into the floor of the sanctuary, a few paces from the Pala d' Oro, there is a heart-shaped slab of porphyry. This marks the spot where Erizzo's vital organ is buried. The porphyry heart is inlaid with two smaller images. One is easily recognisable as the Corno Dogale, the special hat which doges wore, while the black shape is supposed to represent a hedgehog (riccio), symbol of the Erizzo family. 

The rest of Erizzo's body is buried in the church of San Martino, which lies opposite what was once his family palazzo.