288 MISSISSTPPIAN BRACHIOPODA 



obsolete; the cardinal area linear. Internally the valve is thickened 

 towards the cardinal margin, the cardinal process is bifid and from it a 

 pair of crural plates or ridges diverge anteriorly and curve towards the 

 lateral margins of the valve with the hinge sockets excavated from the 

 sides ; the muscle scars occupy a suboval area which does not extend lat- 

 erally beyond the boundaries of the mesial fold and which extends ante- 

 riorly to beyond the middle of the valve. 



Surface of both valves marked by rather strong, concentric lines of 

 growth which are frequently crowded at intervals, especially towards the 

 front. Shell substance finely punctate. 



Remarks. This is one of the most characteristic species in the fauna 

 of the Louisiana, limestone. It may be easily distinguished from other 

 members of the genus by the broad plications on each side of the fold and 

 sinus. On the brachial valve these plications are frequently nearly as 

 wide as the mesial fold itself, while the ones beyond are much narrower. 



Horizon. Louisiana limestone. 



CYRTINA BURLINGTONENSIS Rowley 

 Plate XXXV, Figs. 22-31 



1893. Crytina burlingtonensis Rowley, Am. Geol., vol. 12, p. 308, pi. 14, 



figs. 15-17. 

 1909. Cyrtina burlingtonensis Weller, Bull. Geol. Soc. Am., vol. 20, p. 



310, pi. 13, figs. 20-23. 



Description. Shell small, obliquely subpyramidal in form, the hinge- 

 line usually a little shorter than the greatest width of the shell, and the 

 cardinal extremities a little rounded. The dimensions of two nearly per- 

 fect specimens are : length of pedicle valve 13.2 mm. and 9.6 mm., length 

 of brachial valve 9 mm. and 6.7 mm., width 12.8 mm. and 9.5 mm., thick- 

 ness 9 mm. and 6.5 mm., height of cardinal area 5 mm and 3.3 mm., length 

 of hinge-line 11.6 mm. and 7.5 mm. The dimensions of the holotype are: 

 length of pedicle valve 10.5 mm., length of brachial valve 7.5 mm., great- 

 est width 11.1 mm., thickness 7.2 mm., height of cardinal area 3.6 mm., 

 length of hinge-line 8.5 mm. 



Pedicle valve very deep, obliquely subpyramidal, the lateral slopes con- 

 vex to the cardinal extremities ; the mesial sinus shallow and narrow, orig- 

 inating at the beak, rounded or subangular in the bottom ; the beak sharply 

 pointed and incurved over the cardinal area ; cardinal area high, trian- 

 gular, usually strongly arched with the curvature becoming greater 

 towards the beak, the lateral margins not sharply defined but rounding 

 regularly into the lateral slopes of the valve; the delthyrium high and 

 narrow, covered by a convex pseudodeltidium which is pierced by a 

 rather small, longitudinally subelliptical foramen situated close under the 

 beak, or in some cases apparently imperforate ; each lateral slope of the 



