Form and evolution in the anomiacea (mollusca: bivalvia)-pododesmus, anomia, patro, enigmonia (anomiidae): placunanomia, Placuna (placunidae fam. nov.)

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Abstract
A comprehensive survey of the superfamily Anomiacea is based on an initial study of Pododesmus which proves to be the most primitive living genus. Retention of the genus Monia cannot be justified, its species should be associated with those of Pododesmus. The evolution of this highly specialized superfamily is traced by way of stages represented by 'anisomyarian' heteromyarians such as the modern Mytilacea and then monomyarians inclining to the right like the modern Pteriacea. Complete twisting of the hypertrophied byssal apparatus to the right accompanied by development of a deep byssal notch in that valve results in the assumption of a pleurothetic habit with intimate attachment by way of a calcified byssus. The left posterior byssal retractor displaces the original adductor which assumes a restricted role in shell closure. The presence of the deep notch immediately anterior to the ligament (its position due to the monomyarian condition which involves loss of the anterior regions of the body) is responsible for major asymmetries in the ligamental region. The resilifer on the right valve becomes restricted to a stalked crurum, the corresponding surface on the other valve being sessile. So that it may extend over the smaller under valve, the left valve is increased dorsally as a result of supradorsal extensions of the mantle lobes at both ends of the ligament. An extent of fused shell displaces the umbo from its marginal position. Supradorsal shell secretion on the right valve does not extend that valve beyond the dorsal surface of the crurum. The final effect is the loss of a true marginal hinge, the left valve being pulled downward over the smaller right valve, the ligament vertically instead of laterally disposed. This supradorsal extension involves the epithelia which secrete the anterior and posterior outer ligament layers which now curl back over the dorsal side of the inner ligament layer formed by the mantle isthmus. In Pododesmus there is no fusion of these supradorsal pallial extensions, although the shell they secrete does fuse. Later the tissues withdraw and the exposed upper surface of the ligament breaks down together with the supradorsal area of fused shell on the left valve. The manner in which the very dissimilar valves develop is followed. Except in very young, unattached animals, the long, bulbous-ended and very mobile foot is exclusively concerned with cleansing. In addition to the hypertrophied left posterior retractor, there are small right and left anterior retractors, the former attached to a depression at the base of the somewhat convex crurum. Pallial tissue extends dorsally under this to form a characteristically upward pointing subcrural groove. The pallial organs and ciliary currents are described. The ctenidia are separated anteriorly by the intervening mass of the byssal apparatus, the right proximal oral groove prolonged to pass round this to the mouth. There are very large hypobranchial glands. Heteranomia is distinctive in the possession of unreflected ctenidial lamellae and in the complete fusion of the supradorsal pallial extensions. The reduction of the already small area of union between its mantle lobes involves the secondary extension of this by fusion of the inner folds of the mantle margins above the ligamental area and extending over much of the exhalant chamber. A subcrural groove is present but is usually reduced and rounded. There are no hypobranchial glands and no anterior right retractor, the crurum is horizontal. Anomia resembles Heteranomia in the last respects, although there is no subcrural groove, and in complete supradorsal fusion. But the ctenidial lamellae are reflected and the ctenidia are asymmetrically divided anteriorly, three demibranchs being associated with the left pair of labial palps and only one with the other. Secondary pallial attachment dorsally is exclusively by way of the inner mantle fold of the right side which fuses with the mantle surface within the line of the corresponding folds on the left mantle lobe. Patro, of which shells only were available, appears very similar but has a smaller byssal notch and so a more restricted area of attachment probably projecting from the surface to which it is attached. Enigmonia is a bivalve limpet which occurs on mangroves and other coastal vegetation in southeast Asia. It lives immobile on curved surfaces, such as stems, but also as flat mobile individuals on the lowest leaves of these trees. Only the latter were studied. This is a highly modified anomiid differing from Anomia in the elongate form and in the absence of calcification in the byssal secretion although this remains of supreme significance for 'temporary' attachment which may extend for periods of up to 10 days when some of these animals may be continuously exposed. The foot becomes again an organ of locomotion pulling the animal along, anterior end foremost, by means of a long creeping sole. It terminates in a unique, probably sensory, flagellum. The general surface (not the margin) of the left mantle lobe, covered by the thin, very translucent upper valve, carries a variable number of very simple 'eyes'. The genera Pododesmus, Heteranomia, Anomia with Patro and probably allied genera not seen, together with Enigmonia are regarded as constituting the family Anomiidae, the contrasting characters of the genera being listed in table 1. A small number of shells with only one small preserved specimen were all that were available for the study of the most interesting genus Placunanomia found exclusively off the Pacific coast of central America from Baja California to Equador. The typically plicated shell is much thicker than that of other Anomiacea. Cementation to the substrate must stop at a stage when the right valve ceases to be adpressed to the substrate and begins to assume the adult form. The walls of the opening into the byssal notch then unite and the calcareous byssal plug fuses with the surrounding margin...

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