The process of interface pattern formation in a complex crystal structure is examined through directional solidification experiments in the tert butyl alcohol (TBA) - water system. The transition from a planar interface to cellular and needle morphologies with increasing velocity is examined. The anisotropic effects in the complex crystal structure have been shown to exhibit new features that have not been observed previously. The anisotropy effects have been found to preclude cell elimination and cell creation by the tip splitting processes, so that the wavelength of the cellular pattern is determined by the initial wavelength of perturbation on a planar interface. At higher velocities, a cellular structure is found to transform to a needle structure with highly faceted sides which stabilize the interface against sidebranch instability. The tip region of the needle morphology also exhibits a unique structure that resembles a fingernail, whose formation is governed by the anisotropic properties of the advancing interface. In this complex crystal structure, the growing planes are not symmetrical and the growth occurs preferentially only along specific orientations of the advancing interface which give rise to a fingernail morphology. The scaling law for the primary spacing and the variation of interface temperature with velocity have been found to show a behavior analogous to that for isotropic materials.