EXTERNAL MORPHOLOGY 



SIZE 



Evolution. — Genera in the Stenoscismatacea undergo progressive 

 but irregular increase in size from the Devonian through the Permian. 

 The Devonian genus Atribonium is small, and none of its species is as 

 large as the smallest species of Stenoscisma. However, most species of 

 Atribonium are at least as large as, and many are larger than, the 

 Pennsylvanian species Coledium bozvsheri (Cooper). The smooth 

 Pennsylvanian genus Psilocamara Cooper is smaller than the smooth 

 (but sulcate) Permian Camarophorina Licharev. The trend toward in- 

 crease in size among the Stenoscismatacea, therefore, is definite, but 

 not without reversals throughout the history of the group. 



COMMISSURE 



Description and evolution. — The manner of meeting of the two 

 valves is important in the Stenoscismatacea. Normally the valves butt 

 against one another at the anterior and anterolateral margins, and 

 overlap one another at the posterolateral margins. Two evolutionary 

 changes in the manner of meeting are apparent. In Devonian Atri- 

 bonium the valves tend to butt all the way around the commissure 

 (except, of course, at the hinge). The butting at the posterior slopes is 

 modified by beveling of the valve edges, so that there is slight extension 

 of the outer edge of the brachial valve over the outer edge of the pedicle 

 valve. This is a flange effect rather than actual overlap of the valves. 

 The flanging is more pronounced in the primarily Mississippian genera 

 Coledium and Sedenticellula. The flanged juncture of the postero- 

 lateral margins continues in the Permian genus Torynechus, and also 

 occurs in some species of Stenoscisma. In most species of Stenoscisma, 

 and in Pennsylvanian species of Coledium, the posterolateral slope of 

 the pedicle valve is flat and beveled toward the margin, with the cor- 

 responding portion of the brachial valve overlapping it, sliding over it 

 as the shell opens and closes. This overlapping of the posterolateral 

 valve edges is especially pronounced in the Mississippian to Permian 

 genus Camerisma n. gen. (see Ivanova, 1949, p. 109, fig. 30), and the 

 Permian genus Cyrolexis n. gen. (figs. 18, 19) . 



The anterolateral and anterior margins of the valves butt without 

 overlap in all genera of the Stenoscismatacea; ho\yever, the angle of 

 their meeting differs in different genera. In the primarily Devonian 

 Atribonium the valves flex abruptly toward one another at the anterior, 

 meeting at nearly a straight angle in adults (they meet at any angle, 



