NO. 2 BRACHIOPOD SUPERFAMILY STENOSCISMATACEA — GRANT 39 



pletely elevated in some specimens, partly sessile in others ; an inter- 

 camarophorial plate is present in the brachial valve ; the camarophorium 

 is elevated, intersecting the floor only at its origin in the apex of the 

 beak, and curving ventrally only slightly throughout its length. 



Discussion. — No known specimen of Atribonium has a stolidium 

 such as that in Stenoscisma, but the commissure on some adults pro- 

 trudes slightly as though anticipating development of that structure 

 later in the history of the superfamily. This modification of the valve 

 edges suggests that Atribonium is ancestral to Stenoscisma. The spo- 

 radically partly sessile spondylium may indicate relationship with the 

 Mississippian Sedenticellula in which the spondylium also is sessile in 

 some species. The species of Atribonium that are more bulbous than 

 wedge-shaped in profile (e.g., A. gregeri (Branson)), with the ante- 

 rior margin less flattened than is typical, resemble the type species 

 Sedenticellula hamburgensis (Weller) in that respect. The bulbous 

 forms also have the costae beginning somewhat farther to the poste- 

 rior, indicating possible ancestry to the Mississippian form. Juveniles 

 of Atribonium have the valves meeting at an acute angle as in Sedenti- 

 cellula sacra n. sp., also indicating a possible link between the two 

 genera. 



The intercamarophorial plate of Atribonium is duplex, with a nar- 

 row band separating the two halves. Each side appears to be derived 

 from the underside of the hinge plate, and to extend to the top of the 

 camarophorium, there to fuse. This conclusion agrees with that of 

 Kozlowski (1929) and Licharev (1936) that the intercamarophorial 

 plate is independent of the median septum of the camarophorium. The 

 camarophorium itself begins as a swelling at the top of the septum in 

 the apical part of the valve, supporting the opinion of the above two 

 authors that the camarophorium formed by modification of the ventral 

 edge of the median septum, not from the hinge plate or the crura. 



Growth. — Small juvenile shells of Atribonium lack the flattened 

 anterior margin. The valves meet at an acute angle rather than butting 

 directly nearly in one plane as in most adults. Flattening normally be- 

 gins when the valve has attained a length of 5-7 mm., but some speci- 

 mens retain their juvenile characteristics to normal adult size. Most 

 adults are 10-15 mm. in length, much of the growth of 10 mm. or 

 larger shells involves increase in volume of the shell by increasing its 

 thickness rather than its length. 



The pedicle beak of Atribonium is not strongly curved and does not 

 press against the brachial umbo. The foramen, therefore, is not re- 

 duced or pinched drastically as in some of the late genera of the super- 

 family (e.g., Late Permian species of Stenoscisma). Presumably, the 

 unconstricted foramen contained a functional pedicle in life, and the 



