22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



ing the shell narrowly, possibly against the confining action of sand or 

 mud (in free-living forms) and certainly against the force exerted by 

 the water against motion by the broad stolidia in Stenoscisma. 



HINGE PLATE AND CARDINAL PROCESS 



Description. — The hinge plate in all genera of the Stenoscismatacea 

 is typically rhynchonelloid, supporting inclusion of that superfamily 

 within the Rhynchonelloidea. The hinge plate varies in length, but 

 normally it is rather flat, with a fimbriate swelling at the apex, and a 

 pair of brachial processes extending from the anterior edge. Normally, 

 the plate is undivided, but it is anteriorly recessed in Cyrolexis, and 

 deeply split in Camarophorinella. 



Paired crural processes extend forward from the anterior edge of 

 the hinge plate. They are slender and bow distally while curving ven- 

 trally, roughly following the edges of the camarophorium. Their coin- 

 cidence with the shape of the camarophorium is only approximate ; they 

 curve more strongly toward the pedicle valve (pi. 21, fig. 13; pi. 22, 

 fig. 3a; pi. 23, fig. 4, 5 ; pi. 24, fig. 10). In most specimens their length 

 is roughly that of the camarophorium, but in many they are slightly 

 shorter or longer ( see discussion of crura, below) . 



Functions. — The brachial attachment for the diductor muscles is 

 not a cardinal process in the sense in which that term applies to mem- 

 bers of the Productoidea or the Strophomenoidea ; rather it is a swell- 

 ing in the apical part of the hinge plate that includes a small part of the 

 apex of the valve. The knob is shallowly fimbriate radially, and there is 

 little doubt that it served as an attachment for muscles that opened the 

 shell. 



CRURA 



Description. — The two processes that extend anteriorly from the 

 hinge plate are here called the crura (following Cooper, 1956a, p. 521) 

 although they are longer and more slender than the crura of many 

 other rhynchonelloids. They extend forward about the same distance 

 as the crest of the camarophorium and their curvature reflects approxi- 

 mately the course of the edges of the trough. Posteriorly they are sub- 

 parallel, not strongly curved ventrally, and rather near one another. 

 They bow distally toward the anterior, and curve ventrally somewhat 

 more strongly than the trough of the camarophorium, nearly touching 

 the edges of the spondylium in some specimens ; this is shown in figure 

 1 and in plates 22-24. 



Function. — It is doubtful that the crura supported the fleshy lopho- 

 phore. The body of the brachiopod probably occupied the space within 

 and immediately outside the chamber formed by the spondylium and 



