Onion, structure and mode of reproduction of, i. 388 Oniscus, common wood-louse or slater, structure of the, ii. 202 Onoclea, characters of the genus, i. 348 Onoclea sensibilis, characters and habitat of, i. 348 Oolina claxata, shell of, ii. 48 Opacity in liquids and solids synonymous with accord, i. 37 causes of opacity, i. 37 Operculina, structure of, ii. 38, 46 an instance of an individual combined with a common life, ii. 38. Ophioglossaceæ, characters of the group, i. 365 Ophioglossum, characters of the genus, i. 365 Ophioglossum vulgatum, structure and habitat of, i. 365 Ophionyx, structure of, ii. 173 Ophiuridæ, or snake stars, structure of, ii. 172, 173 power of reproducing rays, ii. 173 Ophrys apifera, or bee ophrys, structure and mode of reproduction of, i. 397 Ophrys, fly, structure and mode of reproduction of, i. 396 Opium, alkaloids obtained from, i. 427 almost universal use of, in the East, i. 427 Orange, amount of radiant heat absorbed by the perfume of, i. 44 fructification of the, i. 381 Oranges, fungus of decayed, i. 290 Orbitolite, characters of the, ii. 33 a fossil gigantic one found in Canada, ii. 54 Orbitolites complanatus, structure of, ii. 33, 34 development and varieties of, ii. 35, 36 habitat of, ii. 36 Orchids, their structure, habitat, and mode of reproduction, i. 388-403 theoretical structure of, i. 403 Orchil, whence obtained, i. 303 from what obtained, i. 124 Orchis mascula, structure and mode of reproduction of, i. 389, _et seq._ Orchis pyramidalis, structure and mode of reproduction of, i. 393, _et seq._ Oscillatoria littoralis, structure and motions of, i. 213 Oscillatoria spiralis, motions of, i. 213 Oscillatoriæ, structure, habitat, and motions of, i. 213 reproduction of, i. 214 Osmunda regalis, structure and habitat of, i. 363 Osmundineæ, characters of, i. 363 sporangia of, i. 343 characters of the group, i. 345 Ostrapods, characters of the, ii. 207 Otolites of Gastropoda, ii. 236 Otolites of Thaumantias, ii. 93 Ovulata, siliceous skeleton of, ii. 60 Oxalic acid, former and present modes of making, i. 116 produced by lichens, i. 303 synthetical formation of, i. 424 Oxygen, i. 6-11 effect of the combination of the atoms of oxygen and carbon in combustion, i. 30 absorptive power of ozonized oxygen, i. 43 absorptive power of, i. 39, 41 proportion of oxygen to hydrogen in the composition of water, i. 94 limit to the number of combustions of, with nitrogen, i. 95 weight of the atoms of, in the peroxide of manganese, i. 99 weight of the atoms of, compared with those of hydrogen, i. 99, 100 spectrum analysis of, i. 139 inhalation and exhalation of, by plants, i. 416, 417 Oysters, iodine found in, i. 19