IO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



do with the free living habit of the adult stenoscismatid, it does not 

 necessarily mean that the stolidium supported the shell on the substrate. 

 However, this seems to be the most reasonable hypothesis. 



An alternate explanation is that the stolidium had a function in the 

 sexual maturity of the individual, perhaps as a producer or place of 

 attachment for clusters of eggs, and that its relationship to the con- 

 stricted foramen is entirely coincidental. This idea is supported by 

 large adult specimens with full-grown stolidia that also have the pedicle 

 beak nearly straight and the foramen open as, for example, specimens 

 of 5". venustum (Girty) (pis. 22, 23) . Such specimens are in the minor- 

 ity, but are fairly common. However, presence of a foramen does not 

 necessarily mean that the pedicle was functional or that the shell con- 

 tinued to attach by it. 



Another possibility is that the stolidium did not support the shell, 

 but functioned as a respiratory and/or auxiliary feeding organ when 

 the shell was partly buried. Ivanova (1949, fig. 30) presented a draw- 

 ing in support of her hypothesis that the shell lay with at least the 

 umbonal region of the pedicle valve in the substrate. She cited the 

 great thickness of the part of "Camarophoria" pentameroides Tscher- 

 nyschev in support of her reconstruction. Although silicified shells of 

 Stenoscisma show no great thickening of the pedicle valve, the shell 

 probably would have become partly buried at the posterior after its 

 release from the juvenile attached condition. In such a position the 

 projecting stolidium could gather oxygen, or perhaps trap food by 

 providing a mucus layer or by channeling the in- and ex-currents. In 

 addition, opening and shutting of the shell would cause the two sto- 

 lidia to create currents at the commissure that could either stir in food 

 particles or flush out invading sediment. Setae probably extended from 

 the troughs between the costae at the valve margins, projecting be- 

 tween the stolidia of the two valves. They may account for some of the 

 grooves, although probably those that bifurcate on the surface of the 

 stolidia are continuations of pallial marks within the shell. 



Available evidence does not indicate clearly the function of the 

 stolidium. Its shape is too random and it seems to be too thin to have 

 functioned as a mechanism that enabled the brachiopod to "flutter" 

 through the water like a Pecten. Its pallial markings might have sup- 

 ported the respiratory or reproductive functions, but they also may 

 have merely supplied nourishment to the mantle that produced the 

 stolidium. 



Taxonomic importance. — The presence or absence of the stolidium 

 is a major consideration in dividing the Stenoscismatacea into two 

 families. Genera in which the stolidium is present constitute the family 

 Stenoscismatidae Oehlert (1887) ; they range from the Late Devonian 



