The shell which lodges both these pinnotheres, in the Mediterranean as well as the Atlantic, is a large acephalous mollusc, known under the name of _Jambonneau_ (a small ham or gammon), and which, according to Aristotle, harbours two different kinds of messmates. This illustrious natural philosopher also described a Pontonia (_Pontonia custos_, Guérin--_P. Pyrrhena_, M. Edw.) about an inch and a half long, of a pale rose colour, more or less transparent, and which lives with its companion, the pinnothere, in the cavity of the _Pinna marina_. This is the same animal which a naturalist of the last century named the _Cancer custos_. We have wished to ascertain whether Pliny knew these crustaceans. He has spoken of them in the following terms:--"The Chama is a clumsy animal without eyes, which opens its valves and attracts other fishes, which enter without mistrust, and begin to take their pastime in their new abode. The pinnothere seeing his dwelling invaded by strangers, pinches his host, who immediately closes his valves, and kills one after another these presumptuous visitors, that he may eat them at his leisure." Cuvier did not believe that the pinnothere brought any food to the 19 mollusc, since the latter, in his opinion, lives entirely on sea-water. Other zoologists regard the pinnothere as an intruder whom chance has brought into this mysterious position. Others again consider mussels as acquaintances possessed of a very curious disposition, and that having no eyes, they have interested in their fate this little crab, which is perfectly provided with eyesight. In fact, in common with other crustaceans of his species, he carries on each side of his carapace, at the end of a movable stalk, a charming little globe, provided with some hundreds of eyes, which he can direct upon his prey, as the astronomer turns his telescope on any point of the firmament. These later naturalists consider, in fact, their crab as a living journal which supplies his host with the news of the day. Rumphius, a Dutchman, the first who described the animal of the nautilus, also understood the habits of pinnotheres. In his "Amboinche Rariteit Kamer," published in 1741, he says that these crustaceans inhabit always two kinds of shellfish, the _Pinna_ and the _Chama squamata_. According to him, when these molluscs have attained their growth, one pinnothere (one only at least in the Chama) lives in their interior and does not abandon its lodging till the death of its host. Rumphius regards this crustacean as a faithful guardian, fulfilling the duties of a door-keeper. In 1638 he found actually two sorts of keepers: by the side of a Brachyuron, carrying an embossed buckler, slender in front, he discovered a Macrouron of the length of his finger-nail, of a yellowish orange colour, semi-transparent, with white and very slender claws. It is without doubt the same animal 20 that Mons. Peters, of Berlin, found on the coast of Mozambique, and of