48 



larly convex sides, and in the form of the septa, which are 

 more deeply concave in the middle lobe; this part being nar- 

 rower in proportion to the entire breadth. In the lateral ex- 

 pansions or outer lobes the septa are recurved towards the 

 apex, and gradually approach each other towards the margin ; 

 while in the G. anceps they make a gentle backward curve and 

 terminate on the edge at nearly their full width. 



Geological Formation and Locality. In limestone of the age 

 of the Trenton group, at Plattcville, Wisconsin. 



GENUS TIIECA, Sow. 

 THECA puiMoiiDiALis, (n. s.) 



Description. Shell elongate, gradually tapering to the some- 

 what obtusely pointed apex. Transverse section sub-trian- 

 gular or planoconvex, 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 slightly convex or nearly 

 flat, the opposite side often regularly rounded, sometimes a 

 little angular along the centre. Aperture transverse, the 

 margin on the flat side extended and forming a semi-circular 

 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 mar- 

 gin of the aperture, and also on the flat side by numerous 

 strong undulations following the lines of growth. The shell, 

 where preserved, is apparently phosphoric, having the same 

 texture and appearance as the Lingulce, with which it is asso- 

 ciated; but it is more readily decomposed. It sometimes 

 attains a length of one inch and a quarter. 



Geological Formation and Locality. Potsdam sandstone, 

 Trempeleau, Wisconsin, and Chippewa River. 



GENUS SERPULITES, McLeay. 

 SERPULITES mukchisonia, (n. s.) 



Description. Body elongate, extremely compressed, very 

 gradually tapering to the acutely pointed apex, gently curved 

 throughout its entire length. Both sides (as they are im- 

 bedded in the sandstone,) very depressed convex, with the 

 aperture prolonged on the inner side of the curve, 



Surface of both sides marked by fine transverse lines of 

 growth, and by numerous strong somewhat equidistant undula- 

 tions, parallel to the margin of the aperture. Length of a 



