l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



developed first in the history of the group, and then its plates spread 

 to form the camarophorium, to provide more surface for muscular at- 

 tachment. Solution of this problem awaits discovery of earlier or ances- 

 tral Stenoscismatacea. 



Evolution. — The camarophorium appears fully developed in the 

 earliest known genus of the group, the Devonian Atribonium where it 

 is comparatively short and its crest is not strongly curved ventrally 

 (fig. 4; pi. 1, figs. 8, 9). Subsequent genera of the group have the 

 camarophorium somewhat more strongly curved in longitudinal profile 

 than does Atribonium. That is, the septum of the camarophorium is 

 proportionately higher at its anterior ( see figs. 20, 23 ; pi. 6, fig. 5 ; pi. 

 9, fig. 2b). 



The spoon or trough of the camarophorium normally is narrowly 

 separated from the hinge plate. The two structures are connected in 

 most genera by the intercamarophorial plate. In Psilocamara Cooper, 

 the intercamarophorial plate is absent; in Cyrolexis n. gen., it is 

 absent or very thin and short ; and in Camarophorinella Licharev, the 

 hinge plate is divided medially, no intercamarophorial plate is present, 

 and the edges of the camarophorial trough join the underside of the 

 hinge plate. 



Taxonomic importance. — Presence of the camarophorium in the 

 brachial valve, along with the spondylium of the pedicle valve, is diag- 

 nostic of the superfamily Stenoscismatacea. Spondylia are present in 

 many groups as modifications of dental plates. For example, Geyerella 

 is separated from Meekella among the Orthotetacea by coalescence of 

 dental plates into ventral spondylium. However, the dorsal camaro- 

 phorium is known only in the Stenoscismatacea. 



THE SPONDYLIUM 



Description. — The spondylium of the pedicle valve is formed by con- 

 vergence and fusion of the two dental plates just above the floor of the 

 valve (pi. 6, figs. 4, 4a; pi. 8, fig. 2a; and Kozlowski, 1929). It is a 

 deep boat-shaped trough in the posterior part of the valve, becoming 

 shallow and scoop-shaped anteriorly where the dental plates terminate 

 and only the base persists (pi. 23, figs. 1, 2, 3 ; fig. 2) . 



Evolution. — Normally the spondylium is elevated above the floor by 

 the septum duplex that is formed by coalescence of the dental plates. 

 However, many species contain individuals in which the dental plates 

 extend to the floor of the valve in the extreme apex, but coalesce and 

 are elevated at a distance less than a millimeter to the anterior. Such 

 individuals are most frequent in early genera such as Devonian Atri- 

 bonium, Devonian and Mississippian Coledium, and Mississippian 

 Sedenticellula; the sessile condition of the spondylium continues far- 



