in their case the rib heads are fused into a single synapophysis. Bunzel therefore concluded that the ribs belonged to a giant lizard. In analogy to Mosasaurus, the giant lizard named after the River Maas, he named this lizard Danubiosaurus anceps, after the Danube. The specific name anceps means "double-headed" in Latin, highlighting the, for a lizard, exceptional trait of having double-headed ribs. In fact the ribs were those of Struthiosaurus. In Ankylosauria, the rump is so flat that the upper part of the rib shafts sticks out sideways, which rotates the short tuberculum to the diapophysis, its vertebral contact facet. Many species have been referred to Struthiosaurus, most based on very fragmentary and nondiagnostic material. Three valid species are recognized by paleontologists: S. austriacus Bunzel, 1871, based on holotype PIWU 2349/6; S. transylvanicus Nopcsa, 1915, based on BMNH R4966, a skull and partial skeleton from Romania; and S. languedocensis Garcia and Pereda-Suberbiola, 2003, based on UM2 OLV-D50 A–G CV, a partial skeleton found in 1998 in France. It is the namesake of the nodosaurine tribe Struthiosaurini, members of which are found only in Europe. A number of invalid taxa have been shown to be junior synonyms of Struthiosaurus austriacus, most of them created when Harry Govier Seeley in 1881 revised the Austrian material. They include: Danubiosaurus anceps Bunzel, 1871; Crataeomus pawlowitschii Seeley, 1881; Crataeomus lepidophorus Seeley 1881; Pleuropeltis suessii Seeley, 1881; Rhadinosaurus alcimus Seeley 1881, Hoplosaurus ischyrus Seeley 1881 and Leipsanosaurus noricus Nopcsa, 1918. Another European ankylosaurid, Rhodanosaurus ludguensis Nopcsa, 1929, from Campanian-Maastrichtian-age rocks of southern France, is now regarded as a nomen dubium and referred to Nodosauridae incertae sedis. The three valid species of Struthiosaurus differ from one another in that S. austriacus is smaller than S. transylvanicus and possesses less elongate cervical vertebrae. Also, though the quadrate-paroccipital process contact is fused in S. transylvanicus, it is unfused in S. austriacus. The skull of S. languedocensis is unknown, but the taxon differs from S. transylvanicus in the flatter shape of the dorsal vertebrae. It differs from S. austriacus in the shape of the ischium. (Vickaryous, Maryanska, and Weishampel 2004) Classification Bunzel was very puzzled by the braincase. He knew that it belonged to a reptile instead of a mammal because of a single as opposed to a double-headed occipital condyle. The back of the head was otherwise not very reptilian as it was low, compact, fused and convex