NO. 2 BRACHIOPOD SUPERFAMILY STENOSCISMATACEA — GRANT 5 



acute through straight in juveniles of various stages of growth). In 

 some species the meeting is curved rather than planar, but the butting 

 is direct. Some species, and some individuals within species, have a 

 slight outbending at the very edges of the valves, producing a slight 

 protuberance along the commissure. Although this feature is a matter 

 of individual variation, it seems to herald later development of the 

 stolidium at the valve margins. The same kind of anterior commissure 

 is present in the Permian genus Torynechus, complete with the slight 

 protuberance along the commissure, like an abortive stolidium. This 

 slight protrusion is present in species of other genera in different fami- 

 lies, e.g., Uncinulus velifer Gemmellaro (see discussion of stolidium). 

 In the Permian genus Septacamera, the valves butt as in Torynechus, 

 but lack the protruding commissure. 



In all genera of the Stenoscismatacea except Atribonium, Tory- 

 nechus, and Septacamera, the valves at the anterior meet at angles less 

 than straight, much as in juveniles of species of Atribonium. The most 

 obtuse angle of juncture is in certain species of the primarily Mississip- 

 pian genus Coledium, in which the stolidium first appears. The angle 

 is obtuse in Psilocamara (Pennsylvanian-Permian), Cyrolexis (Penn- 

 sylvanian-Permian) and Camerisma (Mississippian-Permian), but is 

 smaller (obtuse to acute) in most species of Sedenticellula ( Mississip- 

 pian), Stenoscisma (Pennsylvanian-Permian) and Camarophorina 

 (Permian) . In the Stenoscismatidae, the anterior commissure gave rise 

 to the stolidium (q. v.) as an extrusion from the edges of the acutely 

 meeting valve edges. 



Taxonomic importance. — The angle of meeting of the anterior and 

 anterolateral margins of the valves in adult specimens is of generic 

 importance in the earliest Stenoscismatacea. Direct butting of the 

 valves nearly in a plane is an important character of Atribonium. It is 

 of primary importance, along with costation of the flanks and absence 

 of a stolidium, in distiguishing that genus from Coledium and also 

 helps distinguish Atribonium from Mississippian Sedenticellula, al- 

 though bifurcation or intercalation of costae in Sedenticellula is a more 

 consistent difference. Therefore, the angle of meeting of the valves 

 helps distinguish Atribonium from the genera that are most similar to 

 it and occur nearest to it in time. 



Other features distinguish Torynechus and Septacamera of the Per- 

 mian from Atribonium, but the flat anterior surface helps distinguish 

 these two genera from Stenoscisma. However, this character is less 

 important in the, later stenoscismataceans than in those of the Devonian 

 and Mississippian. 



Functional importance. — The angle of meeting of the anterior and 

 anterolateral parts of the commissure probably is related to evolution 



