104 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



Fossils of the Ceratopea Zone 



Dalmanella electro, (Billings) 

 Pleurotomaria ff canadensis Billings 

 Hormotoma artemesia (Billings) 

 Maclurites sordidus (Hall) 

 Ceratopea keithi Ulrich 

 Raphistomina laurentina (Billings) 

 Goniurus caudatus (Billings) 

 Pliomerops salteri (Billings) 

 Isochilina gregaria (Whitfield) 



Turritoma Zone. The next division of the Beekmantown consists of 

 about 575 feet of pure dove and gray laminated magnesian limestone, 

 which contains in its upper part high-spired gastropods with a species 

 of Turritoma apparently confined to these beds. The lower 375 feet of 

 the Turritoma zone is composed of alternating highly magnesian, finely 

 laminated gray limestones and pure gray and pure clove limestone with 

 occasional beds or streaks of fine limestone conglomerate. The basal 75 

 feet' of this portion occasionally exhibits a few fossils of which Syntrophia 

 lateralis and species of Maclurites and Liospira are most often found. 



The association of species called the Turritoma fauna is found only in 

 the upper 200 feet of this division where the fossils usually occur in beds 

 that weather so as to appear riddled with worm borings! Here the fossils 

 are not silicified, but they occur as dolomitic casts, often, however, in 

 good preservation. They are extremely fragile and much care is required 

 to preserve them. Gastropods, particularly the species Turritoma acrea 

 (Billings), are most conspicuous. A number of species of fossils, too 

 imperfectly preserved for recognition, occur in this zone in Maryland; 

 eight species have been identified specifically. The strata with Turritoma 

 are the uppermost fossiliferous rocks of the Beekmantown, but they are 

 followed by 400 feet of finely laminated, gray, interbedded pure and 

 magnesian limestone of the type considered characteristic of the forma- 

 tion as a whole. At the top of these finely laminated beds are sandy cherts 

 and hard, dense white chert marking the top of the Beekmantown. Asso- 

 ciated with these cherts and continuing upward for about 40 feet are 

 great numbers of the secondarily silicified cherts which have assumed 



