In Part I of this series (2012) we described the structure of the biopolymer interlayers found in the shell of the mollusk Haliotis rufescens (the red abalone). There we described how the layers can be viewed as a viscoelastic composite reinforced by a network of chitin fibrils arranged in an often nearly unidirectional architecture. Mechanical testing documented the response to tensile testing of layers removed via demineralization. Herein in Part II we describe a general viscoelastic constitutive model for such layers that may be both transversely isotropic or orthotropic as would follow from the network of nearly aligned chitin fibrils described by Bezares et al. in Part I (2012). Part III of this series will be concerned with applying the models to more fully describing the response of these types of biological membranes to mechanical loading.