Shell small, rounded outline, posterior dorsal margin straight, rostrum very short and truncate; equivalve, but slightly inequilateral, umbones small and bubble-like; shell surface covered in numerous, crowded, thread-like, concentric striae, oblique rostral ridge not present, but striae irregular and confused posteriorly and extend onto rostrum; resilium small, hinge cither lacks teeth, or has an elongate anterior lateral in the right valve only, or has an anterior lateral tooth in both valves. Septum membranous, lacks dorsal attachments to shell and encloses two longitudinal rows of gill filaments, one either side of the foot; posterior palps large and cup-shaped, siphons cuspidariid in form with three dorsal tentacles and four ventral tentacles and a simple vertical partition, pierced by a keyhole slit, the inhalant valve. Dioecious.
Protocuspidaria represents an earlier stage in the evolution of the septum to that shown by Halonympha in that it possesses true gill filaments with skeletal rods, reduced in size and set in a membranous septum. Neither anterior nor posterior dorsal septal muscles are attached to the shell. The palps are as in Halonympha, and the siphons are cuspidariid and not poromyid in form.
There are relatively few specimens in the collections but, although they all look externally similar, three hinge types are represented. The first has no teeth, resembling the subgenus Myonera, while the other two have previously unknown types, one with an anterior lateral tooth in the right valve only, and the other with an anterior lateral tooth in both valves.
Thus, we use anatomical characters to separate three genera in the family Cuspidariidae:
* Cuspidaria, with a muscular septum pierced by four to ten pairs of highly specialized pores, anterior and posterior septal muscles attached to the shell, and small palps;
* Halonympha, with a less muscular septum, pierced by several slits between very reduced gill filaments but which have no skeletal rods, an anterior septal muscle attached to the shell and large posterior palps;
* Protocuspidaria, with a membranous septum with little muscle containing typical gill filaments, with a skeletal rod, no septal muscle attachments to the shell and large posterior palps. These genera can, for convenience of identification, be divided into subgenera on the basis of hinge characters. Protocuspidaria can be divided further into:
(Protocuspidaria) anterior lateral tooth in the right valve only, type species P. (P.) veritya; (Bidentaria) anterior lateral tooth in both valves, type species P. (B.) atlantica; (Edentaria) no teeth in either valve, type species P. (E.) simplis.
There are two species described in the literature whose shells are very similar, but not conspecific, with the present specimens of Protocuspidaria. These are Myonera ruginosa Jeffreys, 1881, which, like Edentaria, has no teeth, and Cuspidaria colopodes Dautzenberg & Fischer, 1897, which, like Bidentaria, has an anterior lateral tooth in both valves. Since the internal anatomy of these two species has never been described, it is impossible to know if they are species of Cuspidaria or Protocuspidaria. There is no other known species with a hinge resembling that of a subgenus of Protocuspidaria.