12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



of the valves in several Devonian and Mississippian species of Atri- 

 bonium protrude slightly instead of butting against one another di- 

 rectly, apparently presaging later development of the stolidium. The 

 earliest species in which the stolidium developed beyond a slight pro- 

 tuberance are species of Coledium that occur in the Upper Mississip- 

 pian. In these the stolidium may have developed only on an individual 

 basis, or perhaps was present only on mature adults as in later species. 

 It is not preserved on all specimens, but because all known specimens 

 of these species are calcareous, none can be etched free to expose the 

 stolidium that might have been present. 



From its beginning as a mere out-pushing of the valve margins in 

 Devonian and Early Mississippian species of Coledium, the stolidium 

 became a definite extension of the valves in Late Mississippian species. 

 Its breadth increased progressively until in Permian species of Steno- 

 scisma it is rather broad and occupies the crests of fold and flanks of 

 many species ; it is continuous around the anterior margins of several 

 species, connecting fold with flanks. The stolidium is greatly reduced 

 in the Permian genus Torynechus, appearing only as a ridge along the 

 anterior commissure. It is completely absent in Permian Septacamera, 

 which is assigned to the Stenoscismatidae because of its apparent rela- 

 tionship to Torynechus. 



Ontogeny. — The stolidium is absent from juvenile specimens shorter 

 than 5 mm. and rare in specimens shorter than 20 mm. Species in 

 genera of the stolidium-bearing Stenoscismatidae range in maximum 

 length from about 20 mm. to more than 40 mm., but normally the 

 stolidium is present only in adult specimens that are at least two-thirds 

 the maximum size of the species. The stolidium does not appear to be 

 an ontogenetically enlarging structure that is small on young adult 

 specimens and larger on mature or gerontic individuals. Rather, it ap- 

 pears full grown on specimens that have it, and as the shell increased 

 in size the stolidium broke off and was replaced by subsequent fully 

 developed stolidia. 



Relationship of foramen to stolidium. — Most specimens in species 

 that have a well-developed stolidium also have the foramen reduced 

 either by deltidial plates or by tight apposition of the two beaks. This 

 relationship is best illustrated in Stenoscisma; all known species have 

 the stolidium on mature specimens, and there is a marked trend among 

 Permian species for the pedicle valve to curve tightly against the 

 brachial valve. This trend is well expressed in species from the Glass 

 Mountains, Tex., where Late Permian (Guadalupe) species with a 

 broad stolidium have the foramen completely closed. The same rela- 

 tionships hold in Late Permian species from Timor (pi. 20). In con- 

 trast, the two genera of Stenoscismatidae in which the stolidium is re- 

 duced or absent, Torynechus and Septacamera, have the pedicle beak 



