Organic Remains. 221 



failed satisfactorily to assign to its place in the system of nature. 

 The genus belongs almost to the dawn of life on our planet, being 

 found in the oldest silurians both in Europe and America. The 

 species here described is a fine discoidal shell, looking like a 

 broken volute from an Ionic column ; but where one might look 

 for the fractured surface, is a curiously marked spiral operculum or 

 lid strongly marked with the apophyses, by which the ancient 

 tenant held fast his door when pressed with danger from without. 

 An ordinary observer might pass this shell as like a Nautilus or 

 a Planorbis ; but its flat lower side, its sunken upper side, and 

 its singular operculum, strike the eye of a conchologist, and are 

 unlike anything in the modern world. At present it is placed 

 near to the Atalcuitce, small thin shells inhabited by a peculiar 

 tribe of sea snails (Ileteropods). But the Maclurea was a thick 

 heavy shell, and its animal, though, perhaps, more like the Ata- 

 lcuitce than other modern creatures, must have differed very ma- 

 terially from them. 



In collections of tropical shells one sometimes sees specimens of 

 the beautiful but fragile Ianthince or violet snails, which swim in 

 immense multitudes on the ocean, floating by means of a singular 

 raft of air vesicles secreted bv the animals, and to the bottom of 

 which their eggs are attached. In the old Silurian seas multitudes 

 of similarly constructed shells are found, probably inhabited by 

 animals of like nature. They are usually, however, in a condition 

 which does not admit of satisfactory examination, except as to the 

 general external form. But in the limestone of Banquette's Rapids 

 the shell has been replaced by silica, and this when exposed by 

 the weathering of the softer enclosing rock or by the action of an 

 acid, represents the original organism as if just picked up on the 

 beach. Many of the beautiful forms thus revealed are represented 

 in this decade. 



Other floaters of that period, but of somewhat higher organiza- 

 tion, are represented by the genus Cyrtoceras, the floats of certain 

 old cuttle fishes, which, perhaps, preyed on their Atlanta-like 

 companions, as they no doubt devoured Paleozoic medusae and 

 other soft creatures, whose remains have perished. Other shells, 

 bivalves of the genus Ctenodonta, humble burrowers and creepers 

 like our Areas and Nuculas, and fortified like them with a lonrr 

 row of interlocking teeth in the hinge, take us down to the oozy 

 bottom of the Paleozoic seas, where also many univalves, not un- 

 like modern Littorinas and Pyramidellids, — the Cyclo?iemce 1 

 and Loxonemce — creep and perhaps feed on sea weeds. 



