6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



of the stolidium in the Stenoscismatidae. The stolidium is present only 

 in genera whose valves meet at an angle less than 180 degrees. How- 

 ever, some species of Sedenticellula and the one known species of 

 Camarophorina have the valves meeting as in stolidium-bearing spe- 

 cies, but they lack the stolidium. 



The manner of meeting of the posterolateral parts of the valves also 

 probably is related to presence of a functional pedicle foramen. Ivanova 

 (1949, p. 110) cited the tightly sealed "valve articulation" (inferen- 

 tially, the posterior articulation) as a character related to unattached 

 living habits. Ivanova's specimens now are believed to belong to 

 Camerisma n. gen., which lacks the stolidium but has strong overlap 

 of the valve edges. Stenoscisma also has the valves strongly overlap- 

 ping at the posterolateral slopes, shows the greatest development of 

 the stolidium, and in some species the pedicle foramen is tightly closed. 

 Ivanova suggested that the tightly overlapping articulation in this part 

 of the shell prevented the intrusion of mud as the shell lay agape on 

 the sea floor. Articulated silicified specimens of Stenoscisma in the 

 U.S. National Museum collection confirm that the valves could open 

 at the anterior, and remain effectively closed at the posterior by this 

 mechanism (pi. 23, figs. 4, 5). 



PLICATION 



Description. — All genera of the Stenoscismatacea are plicate at the 

 anterior; the shell is thus divided into three lobes. In all genera ex- 

 cept Camarophorina Licharev the fold is in the brachial valve and the 

 sulcus is in the pedicle valve, producing a uniplicate shell (Thomson, 

 1927, p. 58, fig. 17). In Camarophorina the fold is on the pedicle valve, 

 producing a sulcate shell. Only Camarophorina and Psilocamara have 

 the median plication entirely unmodified by costae. 



Juveniles of all genera of the superfamily have the anterior commis- 

 sure nearly rectimarginate. The fold or sulcus normally begins more 

 than 5 mm. anterior to the beaks, and is well developed only on adult 

 shells. The median plication is not prominent in any genus of the Steno- 

 scismatacea, in the sense that it is prominent from beaks to anterior 

 margins as in groups like the Spiriferacea ; its shape and strength 

 (either uniplicate or sulcate) are typically rhynchonelloid. 



Taxonomic importance. — Normal uniplication of the brachial valve 

 is not generically significant within the Stenoscismatacea; only the 

 sulcation of Camarophorina is significant. Details of the shape and 

 height of the fold distinguish genera and/or species, but plication itself 

 is not distinctive. 



Evolution. — All genera and species in the Stenoscismatacea are 

 mesially plicate, without consistent modifications of that character 



