FOSSIL MOLLUSKS. Remains of representatives of all the classes of mollusks occur in the lowest Paleozoic strata, having existed from the Cam brian period down to the present time. As they are so abundant, owing to their preservation as fossils (the shells in the absence of the animal being as a rule useful in classification), the mollusks are of great value as time-marks, and serve to distinguish the different formations.
The class which occurs fossil in the earliest (Cambrian) strata, and concerning which there is no doubt, is the Gastropoda. At the base of the Cambrian (Olenellus zone) have been found the shells of such primitive gastropod genera as SceneIla, Stenotheca, Platyeeras, Rhaphistoma. and Pleurotomaria. The last-named genus is re markable for having persisted to the present time, it is still found at great depths in the Atlantic Ocean. More doubtful are the pteropods of the Cambrian, of the genera Ilya,lithos, Forellella, etc., though their shells are much more abun dantly found than those of the Gastropoda proper. Of pelecypods the Cambrian forms are obscure and doubtful, the only ones yet found be ing a tiny little shell (3fodioloides), and For dilla, which may turn out to be the shell of a crustacean; and this group is comparatively rare even in the Ordovician strata. Of cephalo pods the earliest known genus is Volbo•thella, minute orthoceratite detected in the Lower Cam brian rocks of Finland and Esthonia, and in the Saint John group, Nova Scotia.
The Cambrian, Ordovician, and Silurian mol lusks present (except in generic and specific characters) comparatively unimportant differ ences from living forms. The earliest known pelecypods were the forerunners of the Nuenlidic; the earliest gastropods were Pleurotomaria and allied genera, some still living at great depths; the pteropods were in the main similar to ex isting forms, while the earliest cephalopods were the Orthoeeratites. which are straight. uncoiled nautiloids. In tens of millions of years. there fore. the shelled mollusks have been one of the most numerous and characteristic groups of in vertebrates. and became more and more differen tiated and abundant in species. general. and families as time went on; moreover. they are ex tremely abundant in the .)lesozoic and Cenozoic eras, hence they afford unusually favorable data for the study of the phylogeny of both the chief and subordinate gronps. This state of things
was taken advantage of by Alpheus Myatt in studies carried on for nearly forty years, during which he applied the biogenetic law of Fritz Muller and Ii nickel to the cephalopods, showing that the life of the individual during its rise and decline displays phenomena eorrelative with the collective life of the order to which it belongs. In these and similar studies he was followed by Wurtemberger and Neumayn Their works on fossil cephalopods bear directly on such subjects as quick evolution. the effects of changes of en vironment, the action of use and disuse, :feting throughout numberless generations. The facts gleaned from these mollusks also bear direetly on the causes and mode of origin of the olifferent classes not only of mollusks. hut of all other animal groups. The famous researches on the transmutations of the Tertiary shells of Stein Reim by Ililgendo•f and by Hyatt. those of Neu mayr on the successive forms of Paludina, and the studies in the variations and evolution of other types by later authors. prove how valuable the shells of mollusks are in studies of this nature.
For the uses by man of mollusks, as food or urmonent, or money, see tinder PELEcvrimA; (lAsritoronA; PEARL; CLAM OYSTER; SHELL MONEY. Consult: envier. „lh'moires pour sercir a rhistuire ct a rareotoneic des mollusgucs (Paris. 1817) ; Woodward. Manual of the Mol htsea (4th ed., London, 1880) ; Adams; The Gen era of Recent Shells 1853-58) ; hayes. Trait(' f'br.mentoire do conehlaioiogir (Paris, 18:19-57) ; and the new edition, by Sim roth, of the third volume of Bronn's Klassen end ordnungen des Thierreichs; Gould, Invertebrates of Massachusetts (llogton, 1870) ; Fischer. Man ful de conehyho(ugfie (Paris. 18s3-85) ; Pelseneer, trod?tet ion ti n'tade des 111011118(piCg 1900 ; Tryon and Pilsbury. Manual of Conehologn (Philadelphia, 1896) ; Tryon, Normal of Conchol oy y (Philadelphia. 1S79-50) ; Cooke, "The Mol Itasca." in the rambridge Natural Ilimlory. vol. iii. (New York, 1896). Morphological works of ()wen. Gegenhanr. Huxley. Lamarck. `In•ngcl. Lacaze-Duthiers, Bouvier. Bourne, Peck. thering. Brooks, Ryder, Hyatt, with monographs by Dail On deep-sea forms and numerous recent and fos sil groups, by Binney, Verrill, Bush, Conrad, No belt. Pfeiffer. Martens. Reeves, Stearns, Chem nitz. llourguignant, I'ilsbury, and others.