LIFE HABITS 

 PEDUNCULAR ATTACHMENT 



Early Stenoscismatacea had the pedicle foramen open and presum- 

 ably functional, but both families show progressive diminution of the 

 foramen through time. The earliest genus, Devonian Atribonium, has 

 the delthyrium constricted by a pair of small deltidial plates in the 

 normal rhynchonelloid manner, but the foramen is invariably open. 

 Where deltidial plates tend to close the delthyrium, the foramen pierces 

 the apex of the beak of the pedicle valve. Presumably, all species of 

 this genus lived attached throughout life by a functional pedicle. 



The earliest Stenoscismatidae, Late Devonian and Early Mississip- 

 pian species of Coledium, also have the foramen open and presumably 

 functional, but later species have the foramen closed by tight apposition 

 of the two beaks. Diminution of the foramen of Coledium is accom- 

 panied by development of a rudimentary stolidium. This coordinate de- 

 velopment continued in Stenoscisma, where the stolidium reached its 

 greatest development ; Late Permian species have the foramen com- 

 pletely closed by tight curvature of the beak of the pedicle valve onto 

 the umbonal region of the brachial valve. These species apparently 

 broke free of their juvenile peduncular attachment to live on the sub- 

 strate, possibly supported there by the stolidium. 



The two genera of the subfamily Torynechinae, Pennsylvanian and 

 Permian Septacamera and Permian Torynechus, have the stolidium es- 

 sentially absent. It is reduced to a mere ridge in Torynechus and ap- 

 parently completely absent from Septacamera. The beak is nearly 

 straight to suberect in both of these genera, and the delthyrium is 

 sufficiently exposed to provide an open pedicle foramen. These two 

 genera probably reverted to the earlier habit of the superfamily and 

 lived attached by the pedicle throughout life. 



Among the Atriboniidae the trend toward diminution of the pedicle 

 foramen is similar to that in the Stenoscismatidae but no stolidium 

 developed concurrently. The costate genera Sedenticellula of the Mis- 

 sissippian and Camarophorinella of the Permian apparently lived at- 

 tached by the pedicle. The smooth and highly arched Pennsylvanian 

 genus Psilocamara also typically has an open foramen, although it is 

 greatly constricted by deltidial plates and curvature of the pedicle beak. 

 The open foramen suggests that the genus lived attached by the pedi- 

 cle, although the shape of the shell, with its deeply convex pedicle 

 valve and highly vaulted fold indicate possible capability of survival 

 living free on the substrate. 



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