Somalina hottingeri, a new species with partially vacuolate lateral walls is described from the Middle Eocene of Oman. It is distinguished from the only previously recorded species with this wall structure (S. transitorius (Hottinger)) by having equatorial chambers that are relatively low throughout the test. On the basis of this character, it is suggested that S. hottingeri evolved from the Opertorbitolites douvillei Group (redefined here) and that it gave rise to the true somalines. S. transitorius is regarded as arising from O. latimarginalis (ex. O. latimarginalis Group, introduced in this study) but not to have given rise to any other species. Since forms of Somalina with only partially vacuolate walls appear to be confined to the late Early Eocene to early Middle Eocene, it is concluded that the presence of this wall structure provides a useful stratigraphic marker.