SPIRIFER 347 



115 degrees to the plane of the valve, the lateral margins sharply denned, 

 sloping from the beak to the cardinal extremities, the surface vertically 

 striated ; lateral slopes of the valve each bearing from 5 to 8 simple angular 

 plications, 6 or 7 being the most usual number, which become progressive- 

 ly weaker towards the cardinal extremities ; mesial sinus well defined to 

 the beak, becoming more or less profound anteriorly, marked by an angu- 

 lar median plication which originates near the beak and continues to the 

 anterior margin with gradually increasing size, becoming nearly as strong 

 as the larger plications upon the lateral slopes ; in the larger individuals 

 a single faint plication is sometimes present upon each of the lateral 

 slopes of the sinus, originating from the lateral bounding plications. 



Brachial valve with its greatest convexity near the middle, the surface 

 sloping more abruptly to the cardinal margin, sometimes becoming a little 

 compressed towards the cardinal extremities ; mesial fold well defined to 

 the beak, moderately or rather highly elevated anteriorly, marked by a 

 profound median furrow which divides it into two sharply angular pli- 

 cations, occasionally a single much fainter plication is present upon each 

 lateral slope of the fold in the larger individuals; the lateral slopes are 

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

 valve. 



Remarks. This species was originally described from dwarfed individ- 

 uals from the Spergen Hill fauna, the type specimen not exceeding 2.5 mm. 

 in width. It occurs very generally in the Salem limestone, however, and 

 is usually of much larger size than the type. The species is most closely 

 allied to 8. leidyi, in fact it was described by Swallow at one time as a 

 variety of that species, S. leidyi var merimacensis, and Whitfield has ex- 

 pressed the opinion that the two species are not distinct. A careful study 

 by the writer of many specimens of both species has forced the conclusion 

 that the two forms are well worthy of recognition as distinct species, al- 

 though they are doubtless genetically related, S. bifurcatus being the earlier 

 and S. leidyi the later form. In S. bifurcatus the plications are usually 

 less in number and somewhat coarser, and the lateral plications of the 

 fold and sinus are usually obsolete except in the larger individuals. In 

 S. leidyi the lateral plications of the fold and sinus are always present ex- 

 cept in very rare instances. An occasional example of 8. bifurcatus may 

 be almost identical in its characters with some examples of 8. leidyi, but 

 when one compares the normal or average condition of a large number 

 of individuals of both species, there is a distinct difference between the 

 two, a difference which is sufficiently marked to be of specific rank, 

 especially when the time relations of the two are taken into consideration. 



Horizon. Salem limestone. 



