408 Systematic Paleoktology 



angular below; area variable, usually of moderate height, the exterior 

 margins sometimes strictly defined. Dorsal valve very convex towards the 

 umbo : beak incurved, and often elevated above the hinge-line ; a narrow 

 area. Surface marked by from four to six or seven plications on each side 

 of the mesial fold and sinus: plications elevated, abruptly rounded, the 

 depressions subangular towards the margin ; concentrically marked by fine 

 lamellose imbricating strife and finer radiating striae, which cover the 

 entire shell. The interior of the ventral valve shows strong dental 

 lamellas, which are curved backwards beneath the area, and do not extend 

 so low on the shell as its lower margin. Muscular area not defined in the 

 specimen examined." Hall, 1859. 



Length 1.5 cm. ; width 2.3 cm. 



Hall stated that this species resembled S. cydopierus of the Helderberg 

 and that " it may be only a variety of form, resulting from a change of 

 condition in the sediment." Therefore he gave it the name trihulis mean- 

 ing in this case of the tribe of S. cyclopterus. S. trihulis is certainly a 

 direct development from the pauciplicate forms of that species, but it is 

 readily distinguished in having fewer fimbriate lamellae. At present this 

 species is not often found about Cumberland, because most of the collecting 

 is done in the higher Oriskany. In Mr. Andrews's day it was commonly 

 met with in the lower beds of the Upper Oriskany. 



It seems that either 8. trihulis or the more abundantly plicate forms of 

 S. cyclopterus gave rise to S. intermedius. On the other hand, from S. 

 trihulis also developed S. angularis and 8. murchisoni. The development 

 of the fimbriate, simple spincd spirifers is so rapid in the Oriskany that 

 one is at times perplexed to know to which group single individuals shall 

 be referred. The variations of 8. rnurchisoni are in this respect most 

 troublesome. 



Occurrence. — Oriskany Formation, Shriveb j\Iember. Cash Valley 

 and Devil's Backbone, Collier's Eun, Winchester Eoad near Cumberland, 

 Maryland. Eidgely Member. 21st Bridge. In the lowest beds of the 

 Upper Oriskany it was abundant in the quarries now abandoned on Green 

 Street and back of the German Lutheran Church, Cumberland. 



Collections. — American Museum of Natural History, U. S. National 

 Museum. 



