72 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



clearly is a uniting of two dental plates that join to form a septum 

 duplex. The camarophorium is formed by spreading of the upper edges 

 of the two plates that combined make up the dorsal median septum. Or 

 it might be stated that the septum forms by uniting of the two single 

 plates that make up the trough of the camarophorium. The intercama- 

 rophorial plate also is duplex, and serial sections of this species show 

 that it is entirely separate from the camarophorium, with its base in- 

 serted into the camarophorium much as the septa of the spondylium 

 and camarophorium are inserted into the valve floors. 



Genus SEDENTICELLULA Cooper 1942 



Sedenticellula COOPER, G. A., 1942, Journ. Wash. Acad. Sci, vol. 32, No. 8, 

 p. 231. 



Shell small rhynchonelliform, biconvex; outline elongate to trans- 

 versely subelliptical, tear-drop to bean-shaped; commissure weakly 

 uniplicate, with low fold slightly depressed medially in some species, 

 normally rather wide, beginning at least 5 mm. anterior to brachial 

 beak; posterolateral edges of valves butting, not overlapping, gently 

 curved or protruding ; stolidium absent. Costae low, narrow, beginning 

 at beaks or as much as 5 mm. anterior to beaks, simple or increasing 

 anteriorly by intercalation and bifurcation, on flanks as well as fold 

 and sulcus, producing slightly serrate commissure. Growth lines weak, 

 irregularly spaced, somewhat more frequent near margins. 



Pedicle valve flatly convex, swollen in beak region; beak short, 

 bluntly pointed, nearly straight to erect, beak ridges distinct, outlining 

 flattened area resembling interareas; delthyrium small, triangular, 

 normally nearly rilled by brachial beak, deltidial plates not observed; 

 foramen small. 



Brachial valve more strongly convex, maximum convexity near 

 beak and laterally toward flanks, crest of fold nearly flat ; beak bluntly 

 pointed, not prominent, apex covered by beak of pedicle valve. 



Pedicle valve interior with strong, outwardly bowed dental plates, 

 converging, nearly meeting, on floor of apex, floor between plates 

 thickened in some species, forming sessile spondylium, plates meeting 

 anteriorly in other species: spondylium sessile in juveniles and at 

 posterior of adults, raised at anterior end in adults. 



Brachial valve interior with undivided hinge plate in apex ; short 

 crura extending forward from anterior edge of hinge plate ; median 

 septum low in apex, height increasing anteriorly, extending forward 

 about 5 mm. along floor of adult valve ; camarophorium shallowly con- 

 cave, narrow in apex of valve, expanding narrowly toward anterior ; 

 thick, low, intercamarophorial plate bisecting posterior half of camaro- 



