Vörös A. szerk.: Fragmenta Mineralogica Et Palaentologica 10. 1981. (Budapest, 1981)

The arch leading to the alveoli on the processus coronoideus runs down vertically. Lenght of the trigonium retromolare is 2.0 cm, thus the distance between the arch of the pr. coronoideus and the hypoconulld of the M

3 Is relatively great. The sulcus mylohyoldes Is convex on the lingual side of the ramus mandibulae. It is 3.0 cm long, and its continuation turns into the robust tuberositas pterygoides towards the processus angularis. Height of the ramus mandibulae between the processus angularis and coronoldes as well as between the pr. angularis and condylaris is 13-13 cm. The width from the M

3 hypoconulld to the level of the hindmost point of the ramus is 8.8 cm. On the base of the fragment with such a robust processus we can suppose that the corpus mandibulae must have been relatively slim and arched, and not straight. M

3 (Fig. 4): Among the lower teeth in both mandibules only the M proximal fragments remained. The teeth are medially worn, they are rather lophodont than bunodont. The hypoconulid is 0.7 cm wide, Its surface is quadratic-rhombic, undivided. The hypoconid and the protoconid follow each other in paralel wrinkles. Lenght of the more complete left M

3 fragment is 1.9 cm, width 1.3 cm. The tooth might have been 2.3-2.5 cm long and 1.4-1,5 cm wide. Height of the crown of the tooth is 0.9 cm, lenght of the root is about 2.4-2.8 cm. CONCLUSIONS KRETZOI connected Sirenavus hungaricus with Prorastomus sirenoldes Owen, because of the simple construction of the fronto-nasal region, and classified both into the Prorastomidae family set up by SIMPSON in 1932. The new details, known after the 1978 preparation ascertained that Prorastomus and Sirenavus cannot belong to the same family due to fundamental anatomical differences, that is: - mandible of Prorastomus is straight, while that of Sirenavus must have been strongly convex; - pr. coronoideus of Prorastomus protrudes far over the teeth, while that of Sirenavus runs off vertically, and It Is far from the proximal end of M

3

; - the section of the corpus mandibulae shows an "eight-curve" shape in case of Prorastomus while that of Sirenavus is flat, lentlform; - Prorastomus is Bmaller than Sirenavus

. These differences are more significant than similarities: both genera have a primitive frontal r

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i extended cranium and lophodont set of teeth. The revision of the type specimen of Sirenavus hungaricus proved the existence of Sirenia in the Middle-Eocene of the Carpathian Basin, characterized by a very primitive skull construction differrlng from the known Sirenia forms. In spite of the primitive skull, the mandibles are highly specialized. With the full knowledge of the other Hungarian Eocene Sirenia /Paralitherlum tarkanyense Kordos (KORDOS 1975); Protoslren cf. fraasl Abel (KORDOS 1976); Anisosiren pannonica Kordos (KORDOS 1977); Eot herold es sp. (KRETZOI 1953, KORDOS 1978)/, we can conclude that the genesis of Sirenia preceeded the Eocene, and should be supposed to take place about the Cretaceuos-Palaeogene boundary (Paleocene). TASSY (1979) could also demonstrate a very early appearance of Sirenia with his anatomic examinations, proving that Sirenia represent an independent branch of development separated earlier than Proboscidea. Our studies have also proved that In the Eocene a separate gulf of the Tethys existed on the territory of the Carpathian Basin, thus an independent Sirenia gene-centre could evolve here (KORDOS 1978). This centre is characterized by a very strong mosaic-evolution besides the well-known, genetically stable types known from Egypt as well (Protosiren, Eotheroides)

. The most ancient Sirenia of the Carpathian Basin are of the same age as the Egyptian ones. Thus their separation and the degree of interaction cannot by properly evaluated today, due to the lack of sufficient number of finds. On the basis of palaeogeographlcal maps (HEISIG 1979) we can suppose that further evolution of Sirenia could follow different patterns of evolution in the southern, open Tethys and on the territory of the later Paratethys during the Oligocène. REFERENCES HEISIG, K. (1979): Die hypotetische Rolle Siidosteuropas bei den Säugetierwanderungen im Eozän und Ollgozän. - N.Jb.Geol.Paläont.Mh., (2): 83-96. KORDOS, L. (1975): A new upper Eocene sirenian (Paralitherlum tarkanyense n.g.n.sp.) from Felsőtárkány, NE Hungary.- M.Áll. Földtani Int.évi jel. az 1975. évről: 349-367.