A very singular case of commensalism has been recently discovered with regard to a genus of Australian bivalve shells, _Ephippodonta_. This genus is never found except in the burrow of a species of prawn (_Axius plectorhynchus_ Str.). For some reason at present unexplained, the burrow of this particular prawn appears to be exceedingly popular as a habitat for certain bivalves, for, besides two species of _Ephippodonta_, a _Kellia_ and three _Mylitta_ are found there, and there alone. Sometimes the prawn, when the rock is hard, builds a tunnel of mud upon it, at other times it excavates the soft calciferous sandstone. “This burrow is lined with a tenacious brown mud, composed of excrementitious matter; and, in addition to the mud lining, there is always more or less present an orange-coloured sponge which I have never found elsewhere. Upon the mud or sponge, and adhering very closely, are found the _Ephippodonta_. They quickly form a pit-like depression by means of their foot, and appear almost covered by the mud.” During the winter months (March-July) the prawn appears to fill his burrow, possibly as a provision against stormy weather, with large quantities of minced seaweed, underneath which immense numbers of very young _Ephippodonta_ are found living.[182] The extreme flatness of the _Ephippodonta_ must be due to the same cause as the flatness of the _Lepton_ noticed above, namely, the necessity of not impeding or interfering with the lively motions of the prawn. In the case of _Lepton_ the two valves close completely and the shell is still very flat; in _Ephippodonta_, on the other hand, the same result is produced by the valves being opened to their widest possible extent. As in _Entovalva_, a continuation of the mantle covers the outer surface of the shell. =Variation=