Shell from Transformations of the Carditidae

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The mode of life of Carditidae (Mollusca : Bivalvia) may be either of the byssally attached epifaunal type (or upper tier burrower), in which case they have well-developed byssal threads, or of the free-living infaunal type, in which they have a very weak byssuses or completely lack them throughout all their life. Cyclocardia species with a subequilateral shell belong to the former and strongly inequilateral Cardita species with a prosogyrate beak to the latter. Anatomical and ecological studies of the Carditidae suggest that the byssally attached mode of life is more advanced than the free-living type (Yonge, 1969).Cartesian transformations are a simple deformation of from inscribed in a system of rectangular co-ordinates. If a drawing of Cyclocardia ventricosa within a system of Cartesian coordinates is transferred to a system, after compression in the y-direction, in which for the y of the original diagram substituting Y = 9y/10, a drawing which is a very is a very close approximati … More on to the shell form of C. spurca is obtained. C. turgida has a dorso-ventrally compressed shell, which is subject to a reduction of shell height corresponding to Y = 8y/10. Cartesian transformations seem to be applicable to compare related forms in a genus or in closely related genera of a subfamily.The "simple shear" is a deformation described in terms of the oblique axes X and Y. The Y axis is inclined to y axis at an angle rheta. If the drawing of C. ventricosa within a system of Cartesian co-ordinates is transferred to a system in which for the X=x+y^. tan 345^ﾟ and Y=y, a drawing which is a very close approximation to the shell form of Megacardita and Cardites species is obtained. If the original drawing within a system of Cartesian co-ordinates is transferred to a system of combination of the Cartesian deformation and the "simple shear", a drawing which is a very close aproximation to Megacardita turgida is obtained. The simple shear deformation and the combination of the Cartesian deformation and the "simple shear" may be applicable to compare shell forms of species belong to different subfamilies.The Thecaliinae species have a blood chamber at the ventral margin. The shell form is so peculiar that a close approximation is not obtained in terms of transformations of shell from. The result of shell form transformation suggests that the Thecaliinae is most specialized group within the Carditidae. This is also supported by the paleontological evidence that the Thecaliinae species are only known as living forms. Less