TETRACAMERA 215 



of the flattened or concave lateral regions on each side of the beak; 

 mesial fold obsolete ; beak strongly incurved and partially filling the del- 

 thyrium of the opposite valve ; plications entirely similar to those of the 

 opposite valve. 



The minute surface markings consist of fine, obscure, radiating stria? 

 which are usually obsolete because of the condition of preservation of 

 the shell surface, when preserved they can be best seen upon the flattened, 

 postero-lateral portions of the shell. Fine concentric lines of growth 

 are present when the shell is properly preserved, and are subimbricating 

 in character, stronger lines of growth occur at irregular intervals. In 

 old shells which are much flattened anteriorly by reason of the anterior 

 deflection of the valves, and to a less extent in younger individuals, the 

 margins of the valves are deeply incised by the interlocking serrations 

 of the two valves, and parallel with the line of juncture and adjacent 

 to it the lines of growth are usually conspicuously crowded. 



Remarks. This species is a highly characteristic member of the fauna 

 of the Salem limestone. Its only close ally is T. arctirostrata, which is char- 

 acterized by its larger size and by the presence of fine radiating strige 

 upon the surface of the shell. 



Horizon. Salem limestone. 



TETBACAMERA ARCTIROSTRATA (Swallow) 

 Plate XXVIII, Figs. 25-34 



1863. Rhynchonella arctirostrata Swallow, Trans. St. Louis Acad. Sci., 



vol. 2, p. 84. 

 1910. Cama,rophoria arctirostrata Greger, Am. Jour. Sci. (4), vol. 29, 



p. 73, figs. 11-12. 



Description. Shell below medium size, length and width sub-equal or 

 somewhat wider than long, the greatest width in front of the middle, 

 subtriangular in outline with the postero-lateral margins long and nearly 

 straight, the antero-lateral margins rounded, the anterior margin some- 

 what truncated or slightly convex, the line of junction between the valves 

 deeply and sharply serrate. The dimensions of a very perfect example 

 are: length of pedicle valve 15.6 mm., length of brachial valve 14.8 mm., 

 greatest width 15.4 mm., thickness 12.8 mm. 



Pedicle valve gently convex throughout the greater portion of its 

 surface and gently arched from the beak to near the front margin, to- 

 wards the postero-lateral margins the valve is abruptly deflected to the 

 margin, the deflected portion lying in nearly a right angle to the plane 

 of the valve, antero-laterally the deflection is only a little less abrupt 

 but anteriorly it is notably less, although this margin also curves towards 

 the opposite valve until it lies nearly at right angles to the plane of the 

 valve; mesial sinus obscure, sometimes nearly obsolete, when best de- 

 veloped it is broad and only slightly depressed and is restricted to the 



