The cooperative or diffusively coupled growth of multiple phases during solidification is one of the most widely observed and generally important classes of phase transformations in materials. Technologically, low melting temperature and small freezing range contribute to excellent casting fluidity and fine composite structures give rise to favorable properties. Both of these features contribute to the wide application of eutectic alloys in the casting, welding, and soldering of engineered components. Despite the broad-based technological importance, many fundamental questions regarding eutectic solidification remain unanswered, severely limiting our ability to employ computational methods in the prediction of microstructure for the effective design of new materials and processes. At the core of the most persistent questions, lie problems involving multicomponent thermodynamics, solid-liquid and solid-solid interfacial phenomena, morphological stability, chemical and thermal diffusion, and nucleation phenomena. In the current study, pattern selection dynamics in rod eutectics are investigated using systematic directional solidification experiments and phase field simulations. Directional solidification of a succinonitrile-camphor (SCN-DC) transparent alloy in thin slab geometries of various thicknesses reveals two main points. First, a velocity is indentified at which a transition in array basis vectors is observed in specimens with many rows of rods (i.e. bulk). This transition amounts to a 90 degree rotation of the rod array, shifting from alignment of 1st nearest neighbors to alignment of 2^nd nearest neighbors along the slide wall. Second, significant array distortion is observed with decreasing slide thickness, δ, which ultimately leads to a single-row (quasi-3D) morphology where δ/λ is on the order of unity. In our analysis of these observations, we use a geometrical model to describe the rod arrangement as a function of slide thickness, providing excellent agreement with observation, down to this quasi-3D regime. Further experimental investigation of the early stages of growth shows that the mechanisms involved in the initial dynamics are critical to the array development, especially under geometrical constraint. Phase field simulations show that several rod-type eutectic morphologies are stable over different growth/boundary conditions. Normal circular-rod staggered-array morphologies become unstable and give way to distorted rods and lamellar structures with decreasing material thickness. Distended or peanut-shaped rods are also observed under certain conditions. The boundaries of stability for these growth morphologies and the associated dynamics are investigated here.