136 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



Hemiphragma irrasum (Ulrich) 



Helopora divaricata Ulrich 



Rhinidictya fidelis Ulrich 



Pachydictycu cf r. P. robusta 



Escharopora confluens Ulrich 



Graptodictya sp. (reticulate) 



Chasmatopora reticulata (Hall) 



Chasmatopora sublaxa (Ulrich) 



Hebertella borealis (Billings) 



Hebertella vulgaris Raymond 



Hebertella bellarugosa (Conrad) 



Rafinesquina champlainensis Raymond 



Leptaena charlottae Winchell and Schuchert 



Plectambonites sp. 



Camarotoechia plena (Hall) 



Zygospira recurvirostris (Hall) 



Ctenodonta cf. C. gibberula 



Cyrtodonta sp. 



Gonioceras chaziense Ruedemann 



Leperditia cf. L. faculties 



Isochilina cf. /. gracilis 



Bathyurus sp. 



Amphilichas cf. trentonensis 



Tetradium cellulosum Bed. -One of the most widespread Paleozoic 

 formations of North America is the dove limestone of early Black River 

 age known as the Lowville limestone in the New York section. This 

 formation has received various names in different parts of the country, 

 but its fauna, and even its lithology, is unusually constant throughout an 

 "area extending from southern Canada to Alabama and from the Appa- 

 lachians to the Mississippi River. The coral Tetradium cellulosum, dis- 

 tinguished by its colonies composed of small bunches of four-sided tubes 

 each with four septa, is the guide fossil of the formation. In the Cham- 

 bersburg limestone, the Lowville division of the general time scale, or 

 the Tetradium cellulosum bed, as it is here called, is present locally and 

 sometimes reaches a thickness of as much as 200 feet. As indicated in 

 the sections on other pages, it usually consists of fine-grained, dove-colored 

 limestone with some subgranular beds. In southern Pennsylvania the 

 Lowville forms the basal member of the Chambersburg limestone in the 

 area between Chambersburg and the Maryland state line. It is present- 

 also in the Mercersburg belts but in these it rests on the Caryocystites 

 bed. If represented at all in northern Maryland, it is so thin as to be 

 negligible. 



