Mém. Mus. Roy. Hist. Nat. Belgique No. 63: 1-352. 10 tabs. 52 figs. 11 pls. Dec. 31, 1934.

–One of the most valuable works on fossil sirs., this massive monograph was conceived as a continuation of Abel's (1913a) work on the skull of Eotherium aegyptiacum. Finding himself unable to complete his projected study of Mediterranean Eocene sirs., Abel turned the project over to his student Sickenberg, who in this work describes the postcrania of E. aegyptiacum; covers in detail the other Egyptian Eocene forms E. (Eosiren) abeli, n.sp., E. (Eosiren) libycum, E. stromeri, n.sp., and Protosiren fraasi; Prototherium veronense (Eocene, Italy); Protosiren(?) dubia (Eocene, France); and fragmentary Eocene sirs. from Transylvania and Hungary. In the second part he describes the fossil sirs. of Belgium: Halitherium schinzi forma delheidi (Oligocene), a new combination based on Hartlaub's Manatherium; and Miosiren kocki (Miocene). Sickenberg thereby filled in some significant gaps in the sir. record by his meticulous descriptions of several taxa that had previously been little more than names.