MARYLAND GEOLOGICAL SURVEY 141 



rence of the characteristic fossil of the Nidulites bed, Nidulites pyri- 

 formis, a pear-shaped organism one to two inches long, with the surface 

 marked by sharp ridges forming polygonal spaces a millimeter in diam- 

 eter. This fossil does not reappear in the section until the Greencastle 

 member is reached, but the latter has not been identified in Maryland. 

 These Nidulites are found throughout the 200 to 300 feet of thickness of 

 the bed, but they are most abundant near the middle and the top. The 

 different aspects of this fossil as seen on weathered surfaces of the lime- 

 stone are shown on plate XLVI. Associated with the Nidulites and in 

 equal abundance is a peculiar hemispherical or subglobular bryozoan-like 

 organism described as Diplotrypa ? appalachia in this volume. 



A fauna of about 40 species has been collected and identified by Ulrich 

 from the excellent exposures of the Nidulites bed in Pennsylvania, but the 

 outcrops in Maryland have afforded only a portion of these. With the 

 exception of the exposures of the bed along the bluffs overlooking Conoco- 

 cheague Creek at Wilson, Maryland, its outcrops in the state are usually 

 weathered edges of the highly tilted strata which do not as a rule afford 

 good fossils. Cross-sections of the Nidulites and Diplotrypa can, however, 

 be found in practically every exposure. So far as is known this bed and 

 the overlying Christiania and Greencastle beds are not represented in the 

 geological column elsewhere in North America, As the Sinuites bed at the 

 base of the Martinsburg shales contain a fauna of early Trenton age 

 and as the Echinospherites bed underlying the Nidulites bed contains an 

 assemblage of species much like the upper part of the Decorah shales of 

 the upper Black River, these three intervening divisions should be of latest 

 Black Eiver or earliest Trenton. In terms of the New York section these 

 three divisions would represent an age between the top of the Black River 

 and the base of the Trenton. The species marked with an asterisk are 

 described in this volume. 



Fauna of the Nidulites Bed, Chambersburg Limestone 



Palaeophycus sp. 

 *Nidulites pyriformis Bassler 

 New genus of Amygdalocystidae 

 New genus of Pleurocystidae 

 Bolboporites sp. 



