412 Systematic Paleoxtology 



few strong ridges, and in others with finer longitudinal striae, and more 

 rarely the curving transverse strige are preserved. The cast is strongly 

 papillose on each side of the muscular area. The cast of the dorsal valve 

 shows a sharp median line down the fold, indicating the interior median 

 ridge." Hall 1859. 



Length ahout 4 cm. ; width about 5 cm. 



In New York S. murchisoni is a common shell, fairly constant in its 

 characters, and is one of the diagnostic fossils of the typical or Upper 

 Oriskany. In Maryland the species is very variable, less abundant, and is 

 usually smaller, though one specimen in Mr. Boeder's collection measures 

 along the hinge-line 7.5 cm. The variations are along several lines, as 

 transversity, gibbosity, increase in the width and depth of the sinus, and 

 extreme tongue-shaped extension into the dorsal fold, but mainly in the 

 character and abundance of the plications. These may be few in number, 

 low and rounded, or subangular and even sharply angular when the fold 

 and sinus are also decidedly angulated ; or there may be more plications 

 than is normal when they are subangulated (var. marylandicus) . 



In Washington County S. murcJiisoni almost shades into S. intermedins, 

 and the two species are not readily distinguished. However, the S. inter- 

 medins of this region has less plications than the typical forms, and it may 

 be that larger collections will show that it, ;S'. angularis, S. murchisoni, 

 and S. murchisoni marylandicus are but stratigraphic and local expressions 

 of one variable species — S. murchisoni. Granting this, it would not be 

 desirable to recognize all these forms by one name, and some are here re- 

 garded as species because no intermediate links occur. It is among these 

 plastic forms persisting through great thickness of sediments that the best 

 examples of gradual change through local variations are found. These 

 variations in other localities may become fixed, and are then the easily 

 recognized species. The Spirifers of the Oriskany of the Maryland 

 region express rapid evolution, and some of the fixed forms are found in 

 the Onondaga. 



The name S. murchisonianus de Koninck (d'Omalius, Geology, 1843) 

 is sufficiently distinct from S. murchisoni to be retained. Further, the 

 former is also regarded by Davidson as a synonym for S. disjunctus. 



