Comparisons of endocranial casts of the subfamily Hydrodamalinae (Mammalia; Sirenia) reveal some interesting evolutionary trends, especially in the changes to the shape of the cerebrum and olfactory bulbs, and the position of the optic nerve. Endocranial casts of both Hydrodamalis spissa (Furusawa, 1988 and H. gigas (Zimmermann, 1780) reveal several synapomorphies (spherical olfactory bulbs, the optic nerve separate from the trigeminal nerve, and diminished fissura longitudinalis cerebri and transverse sulcus). A revised phylogeny of North Pacific Sirenia based in part on endocranial casts demonstrates that H. spissa and H. gigas are very closely related within the Hydrodamalinae. On the other hand, although they contemporaneously inhabited the North Pacific Ocean, the eastern Pacific H. cuestae differs clearly from the western Pacific H. spissa