NO. 2 BRACHIOPOD SUPERFAMILY STENOSCISMATACEA — GRANT 21 



Adductor muscles typically unite and attach singly along the median 

 line in the pedicle valve of brachiopods. An elongate area like a muscle 

 mark was observed along the midline of the floor of the spondylium in 

 several silicified valves of Stenoscisma. This area is narrow, but seems 

 to cover sufficient surface to accommodate the united adductor, which 

 is proportionately small in most brachiopods. The remainder of the 

 inner surface of the spondylium probably seated the proximal end of 

 the pedicle, the pedicle adjustor muscles, and the paired ventral ends 

 of the diductor muscle. 



The diductor muscles attached to the fimbriate, knoblike cardinal 

 process on the hinge plate near the apex of the brachial valve. This 

 seat for muscular attachment has been observed in all groups in the 

 Stenoscismatacea. It is especially large and deeply grooved in silicified 

 Permian specimens of Stenoscisma, where it can be observed directly 

 rather than in cross-section. It is large enough in all genera to have 

 accommodated a large and powerful set of muscles. Normally the di- 

 ductor divides to form two muscle marks in the pedicle valve. In this 

 superfamily they apparently attached to the inside surfaces of the 

 dental plates that form the spondylium. 



The muscle arrangement described here is illustrated in figure 1. It 

 provides a set of very short adductor muscles and a longer diductor 

 than in Davidson's arrangement. The camarophorium served as a long 

 lever for the closing of the shell and, although it appears to be a fragile 

 platform, surprisingly great strength is indicated by its frequency of 

 preservation in fossil shells. All specimens that were sectioned serially 

 had the camarophorium intact, and this could not be determined prior 

 to making the section. A great majority of specimens in the large 

 collection of silicified Permian shells from Texas in the U.S. National 

 Museum contains a complete camarophorium or a large part of one in 

 open shells or single brachial valves where it can be observed. The 

 camarophorium is heavily thickened by secondary shell material in 

 some genera, notably Camerisma; this probably added greatly to its 

 strength. 



The gape is very narrow in articulated specimens of Stenoscisma 

 that could be manipulated, and the close similarity of other shells in 

 the Stenoscismatacea indicates that a narrow gape is characteristic. The 

 short adductors and large lever system postulated by the proposed 

 muscle arrangement probably provided a quick closing action by the 

 striated portions of the adductors and a rather powerful locking action 

 by the smooth portions, if the adductors were thus divided (Rudwick, 

 1961, p. 1021). The long, nearly straight diductor, attached well poste- 

 rior to the hinge teeth, provided a relatively efficient system for open- 



