308 S. WELLEB FAUNA OF THE FERN GLEN FORMATION 



nate extensions. Pedicle valve most prominent on the umbo, somewhat 

 compressed toward the cardinal extremities; beak rather small and in- 

 curved; cardinal area low, concave, the cardinal margin sharply denned; 

 mesial sinus of moderate depth, rounded in the bottom, bounded later- 

 ally by a pair of strong, rounded ribs which originate at the beak and 

 which are prominently elevated above the plications of the lateral slopes ; 

 each lateral slope marked by three or sometimes four clearly recognizable 

 rounded plications beyond those which limit the mesial sinus; they de- 

 crease gradually in size toward the cardinal extremities, and in shells 

 greatly extended along the hinge line one or more additional, exceedingly 

 faint ones may sometimes be detected; the largest of these lateral plica- 

 tions is distinctly smaller than those bounding the mesial sinus; inter- 

 nally the umbonal portion of the pedicle valve is solidified, at least in 

 mature individuals; the diductor muscular impression is strongly de- 

 fined and is divided longitudinally by an angular ridge which becomes 

 more septum-like toward the beak, and which in younger examples with 

 less completely solidified beaks doubtless became a distinct mesial sep- 

 tum. Brachial valve less strongly convex than the pedicle, compressed 

 toward the cardinal extremities; mesial fold but little elevated, rounded, 

 with a faint mesial line which is scarcely a depressed furrow; the fur- 

 rows bounding the mesial fold are somewhat more strongly impressed 

 than the others upon the lateral slopes : the plications on the lateral slopes 

 similar to those of the pedicle valve. Surface of both valves marked by 

 fine, but strong and conspicuous, regular, concentric, sublamellose lines 

 of growth which are distinctly arched toward the beak in passing over 

 the plications of the shell. Punctate shell structure has not been ob- 

 served. 



The dimensions of a somewhat crushed individual, more than usually 

 extended along the hinge line, are : Length, 7 millimeters ; width along 

 hinge line, 24 millimeters: thickness, approximately 6 millimeters. 



Remarks. — This is a common species in the Pern Glen fauna, but no 

 perfectly preserved examples have been observed. The pedicle valves 

 are much the more common, only three brachial valves having been seen. 

 In many specimens only the central portion of the shell, including the 

 beak, is preserved, and in but few examples is the extension of the shell 

 along the hinge line shown. In the internal solidification of the beak 

 of the pedicle valve this species resembles S. solidirostris, but this is a 

 smaller shell, more extended along the hinge line and with fewer plica- 

 tions ; it also lacks the distinct mesial plication of the fold and sinus of 

 that species, and the cardinal area is much smaller. The more dis- 

 tinctive characters of the species seem to be the strong bounding plica- 



