SPIRIFER 321 



pair of broad plications which are much stronger than any others on the 

 valve, marked by a single median plication which originates in the um- 

 bonal region and continues without division to the anterior margin. 



Brachial valve a little less convex than the pedicle, the greatest con- 

 vexity near the middle ; mesial fold narrow, sharply defined by a pair of 

 furrows deeper and wider than those between the plications, but little 

 elevated above the general surface of the valve, divided by a median fur- 

 row originating in the umbonal region which is weaker than those between 

 the plications ; lateral slopes convex antero-posteriorly, curving more ab- 

 ruptly to the cardinal margin, the surface compressed towards the car- 

 dinal extremities, marked by plications similar in form and number to 

 those of the opposite valve. 



None of the specimens observed are in a proper condition of preserva- 

 tion to show the finer markings of the shell, although they consisted of 

 concentric lines. More or less distinct concentric lines of growth are 

 sometimes recognizable upon the casts. 



Remarks. With his original definition of the species 8. biplicatus, Hall 

 illustrated two brachial valves, one of which is from the Chonopectus 

 sandstone at Burlington, Iowa, the other from the higher yellow sand- 

 stone of the Kinderhook section at the same locality. 1 A large number 

 of specimens have been studied in connection with the present investiga- 

 tion and they seem clearly to be representatives of two distinct species. 

 The name 8. biplicatus is restricted to the form from the Chonopectus sand- 

 stone which seems to correspond more closely with the definition of the 

 species, the other form being here named 8. biplicoides. The S. biplicatus 

 is characterized by the distinctly biplicate fold and sinus, while 8. biplicoides 

 has an additional lateral plication upon each lateral slope of the fold and 

 sinus in the full grown shells. The cardinal extremities of 8. biplicatus 

 are also much more extended into slender points than in 8. biplicoides, the 

 length of the extensions, when complete, being equal to one-half the dis- 

 tance along the hinge-line from a point beneath the beak to the extreme 

 end of the hinge-line. The length of the body of the shell is proportion- 

 ately less in 8. biplicatus and the number of plications as seen in the casts 

 is usually smaller, although this last character would perhaps not distin- 

 guish the two forms if the 'shell itself were preserved. 



Horizon. Chonopectus sandstone of the Kinderhook. 



1 The specimen from the higher yellow sandstone is erroneously stated by Hall 

 to be from the oolitic limestone bed of the Kinderhook section at Burlington, but 

 the specimen itself is preserved in the American Museum of Natural History in 

 New York, and is clearly from the upper Yellow sandstone. 



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