  In this paper we investigate a general class of linear viscoelastic models
whose creep and relaxation memory functions are expressed in Laplace domain by
suitable ratios of modified Bessel functions of contiguous order. In time
domain these functions are shown to be expressed by Dirichlet series (that is
infinite Prony series). It follows that the corresponding creep compliance and
relaxation modulus turn out to be characterized by infinite discrete spectra of
retardation and relaxation time respectively. As a matter of fact, we get a
class of viscoelastic models depending on a real parameter $\nu > -1$. Such
models exhibit rheological properties akin to those of a fractional Maxwell
model (of order $1/2$) for short times and of a standard Maxwell model for long
times.
