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. Hinge teeth large relative to symphytium, with swollen bases (not supported by 'dental plates'). Cardinalia with inner and outer hinge plates separated by thick crural bases, inner hinge plates forming a V-shaped septalium supported by median septum. 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. Crura are short, subparallel, and with short and pointed crural processes. Adult brachial loop 'trabecular', supporting a 'plectolophous' lophophore. (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.[2]
