NO. 2 BRACHIOPOD SUPERFAMILY STENOSCISMATACEA — GRANT 9 



Torynechus, and absent from Septacamera. It is absent from the pos- 

 terolateral slopes of all species, and normally is widest on the crest of 

 the fold and the highest parts of the flanks. In most species it is absent 

 or very narrow on the sides of the fold, but in a few speceis of Steno- 

 scisma it is well developed there on some individuals. 



The stolidium may be convex or concave in either direction, or may 

 be slightly wavy, but normally the stolidia of the two valves conform 

 to one another. They bear growth lines whose concentricity appears to 

 be a continuation of the pattern on the valves themselves. Their inner 

 surfaces have pallial markings, normally with one or more sets of 

 bifurcations. Many specimens show evidence of former presence of 

 several stolidia on each valve. These appear to have broken off as the 

 valves grew along their edges, with new stolidia progressively replacing 

 those that were bypassed by lengthening of the shell (pi. 20, fig. 2). 

 This indicates that the stolidia grew much faster than the valves them- 

 selves, and that numerous growth lines on the stolidium correspond to 

 few on the shell. The stolidium seems to have been strengthened or 

 perhaps continually repaired by the mantle as long as it grew from the 

 commissure. In order for the shell to grow, however, the mantle either 

 had to split (branch), retract, or slough off the part along the stoli- 

 dium, thus abandoning previous stolidia in favor of growth of a new 

 pair along the subsequent valve margins. 



Silicified specimens of Stenoscisma from the Permian of West Texas 

 have the stolidium in various positions on the two valves. For example, 

 5\ kalum Stehli (pi. 19, fig. 4) has the stolidium on the fold of the 

 pedicle valve, but the stolidium of the flanks is on the brachial valve. 

 Most other species are like S. venustum (Girty), with a stolidium on 

 fold and flanks of both valves (pis. 22, 23) . 



Function. — The function of the stolidium is a matter for conjecture. 

 Stehli (1954) suggested that it served as a support for the shell as it 

 lay on a soft substrate. This hypothesis was supported indirectly by 

 Ivanova (1949) who gave evidence that Stenoscisma lived free on the 

 sea floor, but who did not mention presence of the stolidium (her 

 specimens probably belong to Camerisma n. gen.). Ivanova's evidence 

 was in the blocking of the pedicle foramen by the beak of the brachial 

 valve. Many specimens from the Permian of Texas and of Timor in 

 the U.S. National Museum collection also have the pedicle foramen 

 completely closed by the strong curvature of the beak of the pedicle 

 valve which presses closely on the brachial umbonal region. This condi- 

 tion normally occurs in large adult specimens that have the stolidium 

 (e.g., 5". purdoni, pi. 20) ; normally the foramen is open and presum- 

 ably functional in immature specimens that lack the stolidium. Al- 

 though this relationship suggests that the stolidium had something to 



