FUNCTIONAL-RELATIONSHIP BETWEEN MUSCLES OF MASTICATION AND THE SKULL WITH ENLARGED ORBIT IN THE BAIKAL SEAL (PHOCA-SIBIRICA)

We expected that the enlarged area of zygomatic arch, one of some skul l characters adapted for enlarged eye, has an influence on form and fu nction of muscles of mastication in the Baikal seal (Phoca sibirica). So, in this species, the Mm. masseter, temporalis, pterygoidei, digast ricus were observed in the macroscopic level. The skull characters rel ated to these muscles were also compared between the Baikal seal and a close-related species, the ringed seal (Phoca hispida). The Mm. masse ter and temporalis were well-developed using the enlarged attachment a rea of zygomatic arch. In contrast, the M. digastricus is suggested to be not so important in the Baikal seal, because the temporal bone is not so developed as in the ringed seal. It is suggested that the Baika l seal has especially developed the Mm. temporalis and masseter using an enlarged area of zygomatic arch among Pusa species. We also suggest that the robust temporal bone is equipped to have the M. digastricus developed as a main retractor of mandibular body in the ringed seal.