  We report the discovery of a repeating photometric signal from a low-mass
member of the Praesepe open cluster that we interpret as a Neptune-sized
transiting planet. The star is JS 183 (HSHJ 163, EPIC 211916756) with $T_{\rm
eff} = 3325\pm100$ K, $M_{*} = 0.44\pm0.04$ $M_{\odot}$, $R_{*} = 0.44\pm0.03$
$R_{\odot}$, and $\log{g_*} = 4.82\pm0.06$. The planet has an orbital period of
10.134588 days and a radius of $R_{P}= 0.32\pm0.02$ $R_J$. Since the star is
faint at $V=16.5$ and $J=13.3$, we are unable to obtain a measured
radial-velocity orbit, but we can constrain the companion mass to below about
1.7 $M_J$, and thus well below the planetary boundary. JS 183b (since
designated as K2-95b) is the second transiting planet found with ${\it K2}$
that resides in a several hundred Myr open cluster; both planets orbit mid-M
dwarf stars and are approximately Neptune-sized. With a well-determined stellar
density from the planetary transit, and with an independently known metallicity
from its cluster membership, JS 183 provides a particularly valuable test of
stellar models at the fully convective boundary. We find that JS 183 is the
lowest-density transit host known at the fully convective boundary, and that
its very low density is consistent with current models of stars just above the
fully convective boundary but in tension with the models just below the fully
convective boundary.
