  Order-disorder phase transition of the ferromagnetic Ising model is
investigated on a series of two-dimensional lattices that have negative
Gaussian curvatures. Exceptional lattice sites of coordination number seven are
distributed on the triangular lattice, where the typical distance between the
nearest exceptional sites is proportional to an integer parameter $n$. Thus,
the corresponding curvature is asymptotically proportional to $- n^{-2}_{~}$.
Spontaneous magnetization and specific heat are calculated by means of the
corner transfer matrix renormalization group method. For all the finite $n$
cases, we observe the mean-field-like phase transition. It is confirmed that
the entanglement entropy at the transition temperature is linear in $(c / 6)
\ln n$, where $c = 1 / 2$ is the central charge of the Ising model. The fact
agrees with the presence of the typical length scale $n$ being proportional to
the curvature radius.
