Probably German, probably 17th century

the shell of the turtle and the crown of the head are removable

Overall, the condition of the ivory is good, with minor dirt and wear to the surface consistent with age. The cap of the skull and the turtle's back are carved separately and can be screwed on. A circular section on the forehead is also carved separately. There is minor stable splitting to the ivory, consistent with the material, in particular to the forehead. There are several small holes, particularly around the proper right side of the face; it is possible that there was a detachable piece of ivory which could be inserted here. There are a few further small holes, including to the temples, to the back, and to the upper left side of the head. The spinal cavity is carved as a screw, suggesting there may be a missing piece of ivory which would be inserted here, probably to attach it to a rosary. There are some small scratches, in particular to the skull cap.
"In response to your inquiry, we are pleased to provide you with a general report of the condition of the property described above. Since we are not professional conservators or restorers, we urge you to consult with a restorer or conservator of your choice who will be better able to provide a detailed, professional report. Prospective buyers should inspect each lot to satisfy themselves as to condition and must understand that any statement made by Sotheby's is merely a subjective, qualified opinion. Prospective buyers should also refer to any Important Notices regarding this sale, which are printed in the Sale Catalogue.
NOTWITHSTANDING THIS REPORT OR ANY DISCUSSIONS CONCERNING A LOT, ALL LOTS ARE OFFERED AND SOLD AS IS" IN ACCORDANCE WITH THE CONDITIONS OF BUSINESS PRINTED IN THE SALE CATALOGUE."

Memento mori were known in Northern European during the Renaissance and became particularly popular in both small works of art and painting in the 17th century. Images of skulls and skeletons  were  reminders of the transitory nature of life and therefore the inevitability of death. This skull was likely attached to a rosary composed of smaller ivory beads and is unusual with its two reliquary compartments with removable, threaded covers.

A rosary incorporating a nearly identical decaying skull was sold in these rooms 8 December 1988, lot 402, and another, also with removable crown of the head, on 14 December 1978, lot 222.