In his description, Brocchi does not fail to observe that this is a very variable species. It is more or less transverse or elongated, and while some specimens are entirely smooth, others present from two to four ribs near the front in both valves, and about a similar number on each of the lateral portion of the shell, but these ribs are short and do not seem to extend far in the direction of the beaks. The ventral valve is gibbous, with a well-defined fold, while in the dorsal valve the sinus is broad and deep. In Brocchi's typical form the shell is smooth, while to the more or less plicated specimens Costa has applied the specific name oi plieato-dentata, but, as was well shown by Philippi (Enum. Moll. Sicil., vol. 2, pi. xviii., fig. 5, 1844), there exists every passage in shape between the more transverse and smooth specimens to those that are more elongated and plicated near their margin. Rh. bipartita is also stated by Meneghini and others to occur in the Middle (?) and Upper Miocene, as well as in the Older Pliocene.