Another adaptive lineage of univalves is represented by bilaterally symmetric ortho-conic or cyrtoconic tergomyans, more or less flattened or tall, with the apex inside the apertural ring. Most of these have a rather large whorl expansion rate and relatively isometric broad aperture, providing stability to the shell on the substrate. In this case, the substrate functioned as a "ventral valve" to protect the animal. Linsley (1978) noted that shell shape is significantly correlated with rate of locomotion. Flattened shells, like Proplina and Kalbiella, had low position of both pressure point and center of gravity (figure 15.2:5, 6). Recent tergomyans with flat shells inhabit quieter environments feeding on detritus. Well-developed radular and pedal muscular scars in the Late Cambrian tergomyan Pilina from North China indicate that it was indeed a clamping and crawling grazer (Yu and Yochelson 1999).

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