Lamellibranchia have proved themselves obnoxious to man in two ways. (I ) In the past the cultivation of oysters and cockles in unsanitary situations near sewage-outfalls has led to serious epidemics of typhoid fever, though better cultivation has practically eliminated this danger in England. (2) The shipworm and its allies have been from time immemorial a source of considerable loss through their destruc tion of the timber of ships, piers, jetties, etc. The damage to ships has, of course, been minimized by the introduction of iron-plated hulls ; but Ter edo is still a cause of trouble to marine engineers and does a considerable amount of damage to submarine wooden structures and much time and money have been devoted to combating this pest.
The recognition of Lamellibranchia in the oldest f ossiliferous rocks is somewhat difficult owing to their unsatisfactory pres ervation and the similarity between La mellibranch shells of a certain kind and those of Ostracoda (Crustacea). Glyptarca which occurs in Upper Cambrian strata is certainly a Lamellibranch but the exact posi tion of Fordilla and Modioloides (Lower Cambrian) which have been referred to this class is very uncertain. In the Ordovician Ctenodonta and Modiolopsis are recogniz able as representatives of the Proto branchia and Filibranchia; but the class is not well represented at this epoch. In the Silurian, however, there is a rich Lamelli branch fauna containing representatives of the chief orders except in the Septibranchs, and the modern Aviculidae are recognizable. In the lake beds of Devonian age (Old Red Sandstone) are found shells which resem ble those of modern freshwater mussels and are referred to the genus Archanodon. These forms and apparently brackish water bivalves are well developed in the Carboniferous and a good many marine genera represented at the present time are recognizable at this period, e.g., Trigonia, Astarte and Lucina.
A marked alteration of the class took place in Triassic and Jurassic times. Many Palaeozoic genera died out and were re placed by other genera which have persisted to the present time (Anatina, Isocardia and Perna). The Pernidae and Astartidae
were particularly well represented in the seas of this date.
Reef-building forms such as Chama and the Rudistae and the large genus Inoceramus are characteristic of the Cretaceous fauna.
In the Tertiary we find a fauna very like that of modern times. The Rudistae and other secon dary families have disappeared and the Anisomyaria (Avicula, Pinna, etc.) shows retrogressive tendencies. Specialized genera such as Pholas, Clavagella and Gastrochaena make their ap pearance and both in the repre sentative genera and in the dis tribution of the latter, the evolu tion of the present fauna is clearly foreshadowed.
It will be seen that very little is to be learnt from fossil remains concerning the origin of the Lam ellibranchia and their relation to other Mollusca and the poverty of well authenticated and satis factorily preserved lamellibranchs in Cambrian strata makes it equally difficult to glean any information from this source as to the relationships of the more primitive members of the class.
A recent study of the Pliocene fauna of the lower Kakegawa beds in Japan has led Makiyama to point out that the late Tertiary Lamellibranchia evolved at a much slower rate than did the Gastropoda of the same horizon. If this observation is in accord ance with other data it will con stitute an interesting confirmation of the impression received after a study of living Lamellibranchia, viz., that owing either to their highly specialized mode of life or to some fundamental constitu tional peculiarity, this class of molluscs seems deficient in or ganic energy when compared with the Gastropoda.