SPIRIFER 311 



the curvature becoming stronger towards the beak, the lateral margins 

 sharply defined, sloping from the beak to the cardinal extremities with a 

 convex curve which becomes more abrupt as it approaches the end, the 

 surface marked by vertical stride and by fine longitudinal lines of growth, 

 hinge-line finely crenulate; lateral slopes convex throughout the greater 

 portion of their area, becoming more or less compressed towards the car- 

 dinal extremities, often with a slight sinuosity passing obliquely from the 

 side of the umbo to just in front of the cardinal extremity, each marked 

 by from 15 to 18 subangular or rounded, mostly simple, but rarely bifur- 

 cating plications which grow progressively smaller towards the cardinal 

 extremities ; mesial sinus sharply defined at the beak and losing but little 

 of its definition anteriorly, it is rather deep and subangular or rounded 

 in the bottom, with a conspicuous lingual prolongation in front, it is 

 marked by a median plication which originates near the beak and con- 

 tinues without division to the anterior margin, each side is marked by 

 three or four lateral plications which originate successively from the inner 

 margins of the lateral bounding plications. 



Brachial valve nearly as convex as the pedicle, most elevated at or near 

 the anterior extremity of the mesial fold, with a narrow, linear cardinal 

 area ; mesial fold sharply defined to the beak by a pair of furrows deeper 

 than those between the other plications, depressed and but slightly ele- 

 vated above the general surface of the valve posteriorly, becoming 

 strongly elevated and sometimes slightly recurved towards the anterior 

 margin, simple at the beak but marked anteriorly by 8 or 10 plications 

 which originate through the successive bifurcation of the original me- 

 dian ridge; lateral slopes convex towards the center of the shell, the 

 curvature more abrupt to the cardinal margin, becoming somewhat com- 

 pressed towards the cardinal extremities. 



The entire surface of the shell is marked by fine, concentric, sublamel- 

 lose lines which are crowded at intervals, especially towards the front, to 

 form more or less conspicuous lines of growth. 



Remarks. This species resembles S. marionensis, but it is much more 

 convex in both valves, with a much more elevated mesial fold and de- 

 pressed sinus anteriorly. The shape of the cardinal area of the pedicle 

 valve is also different, the lateral margins sloping laterally towards the 

 cardinal extremities, while in 8. marionensis the two margins of the area 

 are essentially parallel. The manner of growth of the shell of this species 

 is accountable for the difference in the form of the cardinal area from that 

 of S. marionensis. The length of the hinge-line continues to increase 

 throughout the entire period of growth of the shell, although as the shell 

 approaches maturity, the elongation is proportionally less rapid; at the 

 same time the height of the cardinal area is continually increasing so that 

 the mature shell possesses the broadly subtriangular cardinal area. 



Horizon. Fern Glen formation. 



