. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. 248 ANNALS OF THE SOUTH AFRICAN MUSEUM. Fig. 73. Lystrosaurus declivis. Nat. Mus. No. Skull in dorsal view. Included in this species are several specimens ( , Figs 22, 23, 24, 26) which resemble L. declivis in all respects excepting the nature of the transi- tion between the premaxillary and frontal surfaces. In these individuals the anterior two-thirds of the nasal is not included in the premaxillary plane, but slopes back to the frontonasal ridge, thus separating the frontal and pre
. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. 248 ANNALS OF THE SOUTH AFRICAN MUSEUM. Fig. 73. Lystrosaurus declivis. Nat. Mus. No. Skull in dorsal view. Included in this species are several specimens ( , Figs 22, 23, 24, 26) which resemble L. declivis in all respects excepting the nature of the transi- tion between the premaxillary and frontal surfaces. In these individuals the anterior two-thirds of the nasal is not included in the premaxillary plane, but slopes back to the frontonasal ridge, thus separating the frontal and premaxil- lary surface planes. These specimens are small and the variation may well be due to age differences. As shown below, a similar size-correlated variation is noted in the species L. murrayi. The frontonasal suture is of the oviceps type, with the frontal extending down between the nasal and prefrontal. In a few specimens, however, the frontals protrude forward for a short way between the diverging posterior ends of the nasals. Unlike species such as L. curvatiis, the ventral squamosal flange faces directly forward, so that in a lateral orthoprojection the occipital condyles are visible (Fig. 72). Besides the bosses on the frontals and the longitudinal ridges on the snout, several specimens of L. declivis display sharp ridges on the skull roof behind the frontonasal ridge. Thus in the interfrontal suture is raised considerably to form a median, knife-edged crest, while in 3455 the suture between nasal and prefrontal is similarly elevated. The tusks are strong, with inner wear facets. Lydekker (1890) placed L. alfredi and L. depressus under L. latirostris, but Brink (1951) seems correct in stating that all three of these 'species' belong under L. declivis. All three have frontonasal ridges and strongly developed, longi- tudinally ridged snouts. Examination of the type skull of L. primitivus shows that it is probably a badly distorted L. declivis. The spe
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectnaturalhistory, booky
