Significance of the ligament in the classification of the Bivalvia

The ligament is presented as a mid-dorsal, largely uncalcified, region of the shell initially arising by lateral compression preceding calcification. Muscular attachment of the mantle near to the shell margin would then produce anterior and posterior adductors acting in opposition to the ligament uniting now calcareous valves. The primary ligament consists of superficial periostracum with inner (fibrous) and outer (lamellar) layers formed respectively by the mantle isthmus and the hypertrophied outer surface of the outer mantle fold at each end. Secondary extensions of fused periostracum may lead to formation of entirely separate secondary ligaments. The form of the ligament is a basic characteristic of each superfamily, its potential a major factor in the nature and extent of adaptive radiation within it. Its study leads to the conclusions listed on p. 246.