298 SYSTEMATIC PALEONTOLOGY 



at the sutures; whorls about six, gradually increasing from the apex, 

 ventricose, flattened above, obtusely angular on the outer edge; surface 

 marked by fine striae, which upon close examination, are found to be 

 produced by the imbricating edges of lamellae ; striae undulating, bending 

 backwards from the suture and forward in passing over the edge of the 

 shell; aperture obtusely trigonal, depressed above, slightly expanded 

 beyond the dimensions of the whorl just behind it ; axis hollow, umbilicus 

 broad and deep, extending to the top of the spire." Hall. 



Associated with the large shell of this Maclurites is an operculum 

 which undoubtedly belongs to the species. It is large, heavy, and horn- 

 shaped and has the nucleus twisted to the right. In the inner right-hand 

 corner of the operculum, as may be noted in fig. 15, of pi. XXXIX, there 

 is a long process projecting downward into the shell and forming a place 

 for attachment of muscles. 



Occurrence. STONES EIVER LIMESTONE (Middle division). Many 

 localities in southern Pennsylvania; Pinesburg, etc., Maryland. An 

 abundant and characteristic fossil of the Middle Chazyan from Montreal. 

 Canada, to east Tennessee, the original types coming from the Lake 

 Champlain area. In Tennessee the species is so abundant in the Lenoir 

 limestone that this formation has been termed the Maclurea limestone. 



Collections. Maryland Geological Survey, U. S. National Museum. 



MACLURITES SORDIDUS (Hall) 

 Plate XXXVI, Figs. 1-3 



Maclurea sordida Hall, 1847, Nat. Hist. New York, Pal., vol. i, p. 10, pi. iii, 

 figs. 2, 2a. 



Description, Shell subdiscoidal ; consisting of two to two and a half 

 slightly disconnected whorls rapidly increasing in diameter, strongly 

 rounded on the upper side with deeply sunken apex, flattened on the 

 lower side, and the peripheral edge rather sharply rounded. Aperture 

 semicircular. Shell thick, the surface usually with only faint lines of 

 growth, but sometimes strong, transverse striae, with more distant 

 undulations. The transverse lines of growth curve forward on the lower 

 flattened surface and backward on the rounded upper surface. 



