SPIRIFER 359 



SPIRIFER GREGERI n. sp. 

 Plate LV, Pigs. 1-8 



Description. Shell above medium size or rather large, usually a little 

 longer than wide but sometimes wider than long, the hinge-line shorter 

 than the greatest width, the lateral margins usually slightly sinuate just 

 in front of the obtusely angular cardinal extremities. The dimensions of 

 a nearly complete pedicle valve, a partial internal cast, are : length 55 

 mm., greatest width 53 mm., length of hinge-line 38 mm., convexity 22 

 mm., height of cardinal area 6 mm. 



Pedicle valve subhemispherical, the greatest depth near the middle, the 

 surface curving rather abruptly in all directions from the center of the 

 valve, becoming very slightly compressed towards the cardinal extremi- 

 ties ; beak small, rather strongly incurved ; the cardinal area of moderate 

 height, concave, the curvature increasing towards the beak, the lateral 

 margins sharply defined, sloping gradually from the beak nearly to the 

 cardinal extremities, where they bend abruptly to the hinge-line, truncat- 

 ing the lateral extremities of the area; mesial sinus inconspicuous at the 

 beak, becoming more pronounced anteriorly, it is usually narrow and of 

 moderate depth, rounded in the bottom and not sharply defined laterally, 

 sometimes scarcely more than a mesial flattening of the valve ; the entire 

 surface of the valve, both the lateral slopes and the mesial sinus, marked 

 by depressed rounded plications which bifurcate frequently, they vary in 

 width from less than one to one and one-half millimeters, from 25 to 30 

 or more occupy each lateral slope and about 12 the mesial sinus. 



Brachial valve with the convexity about equal to the pedicle, the great- 

 est depth posterior to the middle, the surface curving rather abruptly to 

 the cardinal margin and more gently to the lateral and anterior margins, 

 slightly compressed towards the cardinal extremities; the cardinal area 

 narrow ; the beak obtusely pointed and moderately incurved ; the mesial 

 fold inconspicuous in the posterior half of the valve, becoming moderately 

 elevated anteriorly, not sharply defined laterally ; the surface of the valve 

 marked by plications entirely similar in form and number to those of the 

 opposite valve. 



The minute surface markings of both valves consist of extremely fine 

 radiating strut 1 which are often entirely obliterated in fossilization. Con- 

 centric lines of growth are present which are unevenly distributed and 

 vary in strength. 



Remarks. This species resembles S. grimesi in some respects, but it is 

 a smaller shell with much more rotund form, especially with a much more 

 conspicuously subhemispherical pedicle valve and a narrower mesial 

 sinus. 



Horizon. Lower Burlington limestone. 



