410 Systematic Paleoxtology 



mesial sinus of moderate depth, rounded. Dorsal valve unknown. Sur- 

 face (of a specimen one and a half inches wide) marked by [eight to] ten 

 depressed plications on each side of the mesial sinus. [Detailed surface 

 markings as in S. tiihulis.'] The dental lamellae are not at all or but 

 slightly thickened : the muscular impression is broad and strongly striated, 

 with sometimes a filling of the cavity of the beak." Hall, 1859. 



When Hall described this species he had " a few imperfect specimens of 

 the ventral valve " and apparently regarded it as intermediate between 

 S. cumherlandiw, and S. murcMsoni; hence, the name, ;S. intermediu.s. 

 In aspect it is intermediate, but not so in development. S. murcliisoni 

 and S. intermedius are derived from S. tribulis, as the former develops 

 angulated plications, while the second form retains the rounded plication 

 of the latter, but increases the number. S. cumberlandia', on the other 

 hand, belongs to another phylum, whose ancestors are not yet determined. 



Length about 3 cm. ; width about 5 cm. 



S. intermedins is distinguished from S. cumberlandia' in being larger, 

 attaining a width of 2| inches, is thicker and therefore has more convex 

 valves, has fewer plications, and a lower and less erect ventral cardinal 

 area. The surface lamellae are also less strongly developed in S. inter- 

 medius, and these terminate in marginal spines, which is not the case in 

 S. cumberlandice. 



In 1813 Schlotlieim described a Spirifer from the Middle Devonian as 

 Terebratulites intermedium, and it is a remarkable coincidence that this 

 should be a shell closely related to Hall's species. Both are fimbriate 

 forms. However, it is regarded by Davidson as synonymous with Spirifer 

 speciosus (Schlotheim). According to the rule of nomenclature, once a 

 synonym always a synonym, Hall's name must be abandoned. However, 

 so long as Schlotheim's name is not in use in Europe, the writer prefers 

 to continue the use of Hall's American name. 



Occurrence. — Oriskany Formation, Ridgely Member. Allegany and 

 Washington counties, Maryland; Keyser, West Virginia; 6 miles north- 

 west of Winchester, Virginia. 



Collections. — Maryland Geological Survey, American Museum of 

 Natural History, U. S. National Museum. 



