Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/369

356 upheaved strata—this defect will be in a great measure remedied by the completion of the 'History of British Crustacea,' by Professor Bell, now in the course of publication. Before the appearance of that work Mr. W. Thompson's 'Report on the Irish Fauna' was the only sound authority. The 'Catalogue of British Annellides' lately published in the 'Annals of Natural History,' by Dr. Johnston, has given us a firm basis on which to found our researches in that difficult department: but the distribution of European Annellides—a most interesting subject—is at present a blank in zoological science. We have every reason to believe that both marine worms and Crustacea exhibit a distribution on our shores very correspondent to that ascribed to the fishes. Thus among the higher Crustacea an extreme southern type is indicated by such forms as Macropodia tenuirostris and Pisa gibsii, and certain other species confined as British to the south-western parts of the Channel:—a southern British type by Maia squinado, Pinnotheres pinnæ, Achæus cranchii and Lithodes maia:—a Celtic and central British type by Macropodia phalangium, Inachus Dorsettensis, Hyas araneus, Portumnus variegatus, Pinnotheres pisum and species of Ebalia and Pagurus—a northern, by Nephrops norvegicus, Porcellana platycheles, and species of Alauna and Cuma—and an oceanic or general North Atlantic, probably by Pagurus Bernhardus, Homerus vulgaris and Palæmon serratus. Among the Cirrhopeda too are found Balanus scoticu—important, as will be seen hereafter, in a geological point of view—characteristic of the northern portion of our seas, and Acasta Montagui of the southern, whilst the pedunculated forms are, with the exception of Scalpellum vulgare, either oceanic and prevalent on the ocean side of our islands, or species introduced by their habits of adhering to the sides of ships and to floating wood. The marine shell-constructing Annelida present some peculiarities of distribution worthy of notice by geologists, such as the prevalence of Serpula serrulata in the deeper regions of the northern provinces and the constant occurrence of species of Ditrupa at great depths in several parts of our seas far from land.

Our knowledge of the species and distribution of the British Mollusca is very complete, and sufficient to enable us to apply it to the elucidation of geological problems with safety and effect. When we consider the perfect state in which the testaceous species are preserved, and the facility of specific identification afforded by their shells, this becomes of great importance. In all questions respecting the age of sedimentary strata, the evidence afforded by the fossilized remains of mollusks must, from its completeness, always take precedence of that derived from any other class of animals. Though our native existing species have been well determined, there is no one work upon them to which the geologist can be referred with safety, nor any comprehensive essay as yet