l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



dental plates join at the floor show similar wedged insertion of the 

 fused plates into the valve floor. The same is true of Ombonia and 

 Geyerella whose dental plates fuse well above the floor to form a dis- 

 tinct spondylium on a septum duplex. These Orthotetacea certainly 

 are remotely related to the Pentameroidea and the Stenoscismatacea, 

 their only similarity is in formation of the spondylium by fusion of 

 dental plates and consequent wedgelike insertion of the resulting 

 septum duplex into the valve floor. Therefore, wedged insertion may 

 mean that the septum is derived from structures above the valve floor 

 (i.e., dental plates or cardinal structures), rather than from the floor 

 itself, and therefore must attach to the floor by insertion. 



The septum of the camarophorium inserts in the same way, indicat- 

 ing formation by tissues independent of the valve floor. This is in con- 

 trast to the low septum and "cruralium" of Cupularostrum (olim 

 Camarotoechia) whose calcareous fibers are continuous with those of 

 the valve floor indicating derivation from the floor and growth toward 

 the hinge plate rather than derivation from the cardinalia (or inde- 

 pendent tissue) and growth toward the floor. 



Kozlowski (1929, p. 131) called the spondylium of Stenoscisma 

 (then Camarophoria) a typical spondylium duplex. By this he meant a 

 spondylium that is formed of two plates that unite with an interseptal 

 lamina between them (1929, p. 125). Licharev (1936, p. 66) clarified 

 this concept as applied to the Stenoscismatacea, emending it to mean 

 a septum formed of two distinct plates. No interseptal lamina was ob- 

 served in any of my serial sections, only the dark line where the two 

 plates of the septum meet. Therefore, the term "duplex" is used here in 

 Licharev's sense, rather than in Kozlowski's (unless by interseptal 

 lamina Kozlowski meant merely a plane of joining) . 



MUSCULATURE 



Muscle marks are not strongly impressed in the Stenoscismatacea; 

 they are invisible in most specimens. However, a few valves in the 

 U.S. National Museum's large collection of silicified specimens of 

 Stenoscisma and Torynechus from the Permian of Texas have light 

 marks of muscle attachment in the concave surfaces of the camarophor- 

 ium and the spondylium. These marks support the contention of King 

 (1850, p. 115) supported by Licharev (1936, p. 65) and Cooper 

 (1956a, p. 521) that the adductor muscles attached within the cama- 

 rophorium (fig. 1). In addition, the inner surfaces of the camarophor- 

 ium and spondylium are smooth in many valves where individual mus- 

 cle marks are not distinguishable. Their smoothness contrasts with the 

 rougher or pitted nature of the valve floors, the sides of the septa, and 

 the convex surfaces of the camarophorium and spondylium. Originally 



