From the Metropolitan Museum of Art; the website explains:
Rosary, ca. 1500–1525
Ivory, silver, partially gilded mounts
Overall: 24 11/16 x 2 1/8 x 1 3/4 in. (62.7 x 5.4 x 4.5 cm) Gift of J. Pierpont Morgan, 1917 (17.190.306)
Each bead of the rosary represents the bust of a well-fed burgher or maiden on one side, and a skeleton on the other. The terminals, even more graphically, show the head of a deceased man, with half the image eaten away from decay. Such images served as reminders that life is fleeting and that leading a virtuous life as a faithful Christian is key to salvation.
You can find out more about this wonderful object by clicking here.