NO. 2 BRACHIOPOD SUPERFAMILY STENOSCISMATACEA — GRANT 13 



nearly straight, and the delthyrium is bounded by small deltidial plates 

 that leave the foramen open. 



The inverse relationship of foramen to stolidium holds only for the 

 Stenoscismatidae. In the Atriboniidae, the same trend toward closing 

 of the foramen is apparent, but no stolidium is present. 



DELTHYRIUM AND DELTIDIAL PLATES 



Description. — The delthyrium is triangular in all species in the 

 Stenoscismatacea. It varies in the manner and extent of constriction by 

 deltidial plates, and by curvature of the pedicle beak over the brachial 

 beak. The delthyrium in Atribonium (Devonian) is constricted in the 

 normal rhynchonelloid manner by a pair of conjunct deltidial plates. 

 In A. cooperorum n. sp. the delthyrium is so constricted by conjunct 

 plates that the foramen would be nearly completely closed were it not 

 for penetration of the foramen into the apex of the beak. In Atribonium 

 the foramen is kept open either by slight constriction by the deltidial 

 plates or by incursion of the foramen into the beak of the pedicle valve. 

 In other genera of the superfamily the delthyrium is either unmodified 

 and unconstricted, constricted only sporadically by deltidial plates, or 

 constricted by tight curvature of the pedicle beak onto the brachial 

 umbonal region. 



Function. — The delthyrium primarily afforded an opening by which 

 the pedicle could emerge from the ventral valve to attach the shell to 

 the substrate. Some specimens of Stenoscisma with disjunct deltidial 

 plates have the foramen nearly or entirely closed by tight apposition of 

 the two beaks. Other specimens, especially of Permian species, have the 

 size of the foramen reduced by the deltidial plates. Apparently the 

 pedicle of many Permian species was not funtional; this conclusion 

 also was reached by Ivanova (1949) from study of Permo-Carbon- 

 iferous specimens (of Camerisma n. gen.) from the Moscow Basin. 



Evolution. — The earliest known stenoscismatacean, Devonian Atri- 

 bonium, has a typically rhynchonelloid delthyrium, constricted by a 

 pair of small, disjunct or conjunct deltidial plates; the foramen ap- 

 pears to have been able to accommodate a pedicle in all individuals of 

 all species. Likewise, no known specimen of Atribonium has a stoli- 

 dium, and none has the valves overlapping at the posterolateral mar- 

 gins. 



Species in subsequent genera of the superfamily (except Camerisma 

 and Cyrolexis) have individuals with the foramen open and others 

 with it closed either by the deltidial plates or by the brachial umbo. 

 This variable condition occurs even in Stenoscisma in which adults of 

 all species invariably bear stolidia. In Camerisma and Cyrolexis, all 

 known specimens have the foramen tightly sealed by apposition of the 

 umbones. 



