[473] Organic Remains of a Former World, vol. iii. See Pictorial Atlas.
[474] Geology of Yorkshire; and Palæozoic Fossils of Devon.
[475] Icones Fossilium Sectiles.
[476] Bridgewater Treatise.
[477] Silurian System, chap, xlvii.
[478] Die Organisation der Trilobiten. 1843. Translation: Ray Society.
[479] In the Quarterly Journal of the Geological Society, and in the Memoirs and Decades of the Geological Survey of Great Britain.
[480] Palæozoic Fossils in the Cambridge Museum.
[481] The Silurian System of Bohemia: the Trilobites.
In the Trilobites the head is distinct, and without antennæ, and the feet are supposed to have been rudimentary, soft, and membranaceous: the essential characters which separate them from all other crustaceans, except Bopyrus (a parasite on the branchiæ of the common prawn), are, according to Mr. Macleay, the deficiency of antennæ, and of lateral posterior abdominal appendages, and the presence of evanescent feet. Like other crustaceans, the Trilobites were subject to the process of metamorphosis during their early stages of life; and M. Barrande has ascertained that one species, the Saö hirsuta, appeal’s in no less than twenty different stages of development. In its earliest, embryonic condition, it is a simple disk, and it passes through various stages until it becomes a perfect adult trilobite, having seventeen free thoracic segments and two caudal joints. No less than ten genera and eighteen species were instituted by palæontologists on some of the forms only which this one species presents in its different stages of metamorphosis, before M. Barrande’s laborious and long-continued investigations gave him an insight into the true relations of these various conditions of the same animal to one another. This talented and indefatigable palæontologist has arrived at like results with other Trilobites, and has been enabled to add greatly to our knowledge of the natural history and geological distribution of this interesting group of crustaceans. See Transact, of the Sections, Brit. Assoc. 1849 and 1850; and Trilob. Bohême, pl. vii.