J. jaffaensis reaches a maximum length of 23.4 mm and fits the following diagnosis: Overall outline ovoid to circular, relatively thin laterally and with commissures (junction between the dorsal and ventral shells/valves) straight, anterior commissure rectimarginate to lightly sulcate.

Beak (posterior point of shell) suberect, with deltidial plates conjunct (as symphytium) and mostly exposed, and bearing a small foramen.

Cardinal process well developed, supported anteriorly by a triangular ridge between the hinge plates, and with a posteriorly facing myophore.

The median septum does not reach mid-valve, highest at junction with lateral connecting bands of brachial loop.

(modified and simplified from[2]) The genus is very similar in morphology to the genus Nipponithyris (sharing a large anteriorly supported cardinal process, thick crural bases, 'trabecular' brachial loop, large and thick based hinge teeth, and rounded shell outline without strong folding) and they may prove synonymous.