An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Theria

Theria

Theria is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians and metatherians . The only omitted extant mammal group is the egg-laying monotremes....

n mammals (marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. Monotreme

Monotreme

Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...

s, although part of the mammalian class, do have interclavicles. In birds, the interclavicle is fused with the clavicles to form the furcula

Furcula

The ' is a forked bone found in birds, formed by the fusion of the two clavicles. In birds, its function is the strengthening of the thoracic skeleton to withstand the rigors of flight....

 (wishbone). In chickens the furcula forms a "Y" shape and the interclavicle is the stem of the "Y".