CONTRIBUTIONS TO PALEONTOLOGY. 135* 



PTEROPODA. 



GENUS THECA (Sowerby). 

 THECA PRIMORDIALIS. 



PLATE VI. FIGS. 30 & 31. 



Theca primordialis : Hall, Annual Report of Progress, Geological Survey of Wisconsin, 



1861, p. 80. 



Geological Report of Wisconsin, 1862, Vol. i, p. 21. 



Pugiunculus : cited in the text of this paper. 



Shell elongate, gradually tapering to the sonnewhat obtusely- 

 pointed apex. Transverse section subtriangular or plano-convex, 

 the diameters about as two to one : length of shell about three 

 and a half times as great as the width of the aperture ; one 

 side very convex or nearly flat ; the opposite side often re- 

 gularly rounded, sometimes a little angular along the centre. 

 Aperture transverse, the margin on the flat side extended and 

 forming a semicircular lip ; that of the convex side transverse 

 to the axis of the shell, with sometimes a slight emargination 

 near the middle. 



Surface marked by fine lines of growth parallel to the margin 

 of the aperture, and also on the flat side b}^ numerous strong 

 undulations following the lines of growth. The shell, where 

 preserved, has the same texture and appearance as the Lingulse 

 with which it is associated ; but it is more readily decomposed. 

 It sometimes attains a length of one inch and a quarter. 



Some specimens of this fossil originally examined, induced me to regard 

 the shell as phosphatic, like the Lingulas ; but subsequent examinations 

 have not verified this opinion. 



This fossil usually occurs as casts of the interior, the shell having been 

 removed. Large numbers of individuals are sometimes found closely crowded 

 together, as if having been drifted by waves or currents. It is associated 

 with Lingula ampla, Obolellaf polita, Conocephalites minor and C. iowensis, 

 at Trempaleau. It likewise occurs near the mouth of Black river, and on 

 the Chippewa river? in Wisconsin. 



