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Show 1886.] DR. GUNTHER ON A FIERASFER IN A PEARL-OYSTER. 319 Tbe specimen, which is represented in the accompanying woodcut of the natural size, has been in my possession for many years. It is an old shell of Margarita margaritifera, in which there is imbedded, behind the impression of the attractor muscle, a perfect individual of a fish belonging to the genus Fierasfer. The fish is covered by a thin layer of pearl-substance, through which not only the general outlines of the body but even the eye and the mouth can be seen. The parasitic habits of Fierasfer are well known, and Putnam describes, in the ' Proceedings of the Boston Society of Natural History,' vol. xvi. 1874, p. 344, a species, Fierasfer dubius, which is found on both coasts of Central America, but inhabits Holothurians on the Atlantic, and Pearl-Oysters on the Pacific side ; and he further mentions, in a footnote, an example belonging to the |