336 MISSISSIPPIAN BRACHIOPODA 



Brachial valve about as convex as the pedicle, the greatest convexity 

 near the middle ; the lateral slopes convex towards the middle part of the 

 valve, the most abrupt curvature being towards the cardinal margin, be- 

 coming somewhat compressed towards the cardinal extremities, marked by 

 plications similar in form and number to those of the opposite valve; 

 mesial fold denned by somewhat stronger furrows posteriorly but scarcely 

 elevated above the general surface of the valve, anteriorly it becomes 

 moderately elevated and is rounded or subangular along the median line 

 and ill-defined laterally, it is marked by plications which are similar in 

 form and number to those of the sinus of the opposite valve, all of which 

 originate from a single plication at the beak. 



The surface of both valves is marked by moderately fine, concentric, 

 sublamellose lines which arch posteriorly as they cross the plications, and 

 also by a few much stronger lines of growth which are usually stronger 

 and more frequent towards the anterior margin. 



Internally, as shown by casts of the interior preserved in chert, the den- 

 tal lamelke of the pedicle valve are short and diverge at an angle of about 

 60 degrees, the muscular impressions occupy a subovate area and are 

 moderately impressed. 



Remarks. This is a rather common species in the fauna of the Bur- 

 lington limestone which has commonly been confused with 8. imbrex. 

 The reason for this confusion is doubtless to be found in the imperfect 

 specimen which was used by Hall as the type of the species, and the single 

 unsatisfactory figure which accompanies the original definition. This 

 type specimen is illustrated herewith, in three views, and, in addition, 

 figures of more complete examples are given. The species differs from 

 S. imbrex in having much finer and less angular plications. 



Horizon. Burlington limestone. 



SPIRIFER CARINATUS Rowley 



Plate XLVI, Figs. 13-17 



3900. Spirifer carinatus Rowley, Am. Geol., vol. 25, 'p. 269, pi. 5, figs. 4-6. 



Description. Shell above medium size, wider than long, the hinge-lino 

 equaling or nearly equaling the greatest width, the cardinal extremities 

 subrectangular. The dimensions of a nearly complete specimpn with the 

 brachial valve somewhat crushed, are: length of pedicle valve 34.5 mm., 

 length of brachial valve 29 mm., greatest width 51.5 mm., thickness 23.5 

 mm., height of cardinal area 6 mm. 



Pedicle valve with its greatest convexity near or posterior to the 

 middle, the fullness of the valve extending well out towards the lateral 

 margins, the surface curving abruptly to the cardinal margin on either 

 side of the beak and a little more gently to the anterior margin, from 

 the central portion of the valve to the lateral margins the curvature is at 



