2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



Devonian Atribonium n. gen. In this family the valves meet directly at 

 the anterior and butt against one another, as in most genera of the 

 Rhynchonelloidea. 



The general form of stenoscismatacean species is rhynchonelloid, 

 with the typically sharp beak and a foramen that occupies a delthyrium 

 normally constricted by deltidial plates. Internally the relationships are 

 somewhat less obvious, but the spondylium in the pedicle valve gen- 

 erally is conceded to be formed by fusion of rhynchonelloid dental 

 plates {e.g., Kozlowski, 1929), and the crura appear rhynchonelloid. 

 The camarophorium is peculiar to the Stenoscismatacea, but serial sec- 

 tions show clearly that it forms by separation of the ventral edges of 

 the two plates of a septum duplex, which, along with its beginning as an 

 apical swelling of the septum, argues for its development from that sep- 

 tum. The camarophorium is a modification of a normal rhynchonelloid 

 median septum. 



The hinge plate is rather conservative in its development through 

 the Stenoscismatacea. Silicified specimens from the Permian of Texas 

 have a finely fimbriate cardinal process, or apical boss, probably for 

 reception of diductor muscles. Serial sections of Devonian and Missis- 

 sippian specimens also show the fine fimbriation, although less clearly. 



The form of the lophophores that extended beyond the calcareous 

 crural processes in the Stenoscismatacea is not known. They may have 

 been helicoid, as in Recent rhynchonelloid genera, but no morphological 

 evidence from the fossils supports this conjecture. The form of the 

 calcareous crura is similar to that of normal rhynchonelloid genera, 

 only longer and more slender, reinforcing evidence from the spondyl- 

 ium and camarophorium that the Stenoscismatacea belong with the 

 Rhynchonelloidea. 



Earlier authors believed that the stenoscismataceans are derived 

 from the Pentameroidea {e.g., Weller, 1914), but Kozlowski (1914 

 and 1929) cited cogent reasons for placing them in the Rhynchonel- 

 loidea. Subsequent authors have concurred because similarities between 

 stenoscismatacean and normally rhynchonelloid dental plates, median 

 septum, hinge plate, crural processes, cardinal process, beak form, 

 delthyrium, foramen, and deltidial plates indicate close relationship. 

 Structures in the Pentameroidea that resemble those of the Stenoscis- 

 matacea are in opposite valves, and upon close inspection bear little 

 resemblance to them. Furthermore, external profile, manner of valve 

 junction, and especially absence of deltidial plates in the Pentameroidea 

 show lack of relationship with the Stenoscismatacea. 



TYPE GENUS 



The superfamily is founded upon the genus Stenoscisma Conrad 

 (1839), senior objective synonym of Camerophoria King (1844, 



