VLA HI Observations of Two Edge-On `Superthin' Spirals by Dr. Lynn Matthews (CfA)   `Superthins' are a subset of spiral galaxies seen edge-on that exhibit highly flattened stellar disks, small disk scale heights, and little or no bulge component. These appear to be among the dynamically least evolved disk galaxies, and therefore can offer important clues on the dynamical heating and evolution of disk systems. Frequently, superthins are seen to have low star formation rates, diffuse stellar disks, and large neutral gas fractions, indicating that these are examples of ordinary low surface brightness (LSB) galaxies viewed at high inclination. Thus superthins also offer a unique opportunity to probe both the radial and vertical structure of the interstellar medium of LSB spirals. I will present results from the analysis of sensitive new HI 21-cm line observations of two superthin spirals observed with the Very Large Array, including an overview of the neutral gas disk structures and kinematics of these galaxies. I will discuss how their HI properties compare with those of other types of spiral disks, and present some new clues as to the role initial conditions versus environment may play in shaping the structural and dynamical properties of the gaseous component of spirals.