**Abstract** : Thick lava flows that are a feature of many volcanic fields on the Earth and Venus vary from sheet-like to nearly perfect axisymmetric domes. Here, we investigate how these geometrical characteristics depend on the shape of the feeder vent. We study the gravitational spreading of viscous lava erupting from elliptical vents onto a flat surface using 3-D numerical models. The aspect ratio of the vent, defined to be the major to minor axes ratio, varies between 1 and 25. In the limit of an aspect ratio of one, the vent is circular and spreading is axisymmetric. In the limit where the ratio is large, the vent behaves as a fissure. The numerical models rely on an isoviscous lava rheology and a constant volumetric eruption rate. In all cases, the initial phase of the dome's evolution is in a lava-discharge dominated regime such that spreading is insignificant and the height of the dome increases at a constant rate over the vent area. For vent aspect ratios greater than five, three successive regimes of spreading are identified: 2-D spreading in the direction perpendicular to the major axis of the vent, a transient phase such that the dome shape evolves towards that of a circular dome and a late axisymmetric spreading phase that does not depend on the vent shape. These regimes are delimited by the times required for the flow thickness above the vent to reach a given height and for the flow to spread axisymmetrically up to a length equal to the semi-major axis of the vent. Numerical results for the flow height and runout length tend towards the similarity solutions in the 2-D and axisymmetric regimes. Two main implications for highly viscous (rhyolitic) fissure eruptions can be drawn. First, the fissure length determines the flow regimes. The longer the vent fissure length, the longer the early lava discharge regime and 2-D spreading perpendicular to the length of the fissure. Second, the aspect ratio of fissure-fed lava flows can be used as an indicator of the fissure length and the duration of lava discharge. The ellipticity of some terrestrial fissure-fed flows provides evidence for viscous gravity-driven spreading terminated before the onset of the axisymmetric regime. On the other hand, the circular domes on Venus appear to be the result of fissure-fed eruptions sustained enough for the spreading to reach the axisymmetric regime. We propose relationships providing estimates of the fissure length and the duration of lava discharge based on fossil dome dimensions.