66 BRACHIOPODS. 



foramen having nearly the form of an equilateral triangle, or 

 sometimes slightly wider than high, or the reverse. Interior 

 with hinge teeth moderately prominent and trigonal ; cavity 

 for the reception of the muscular attachments comparatively 

 small, scarcely reaching the middle of the valve, longer than 

 wide, or elongate-oval, with nearly straight and parallel sides, 

 always well defined by the dental ridges, and on old specimens 

 extremely profound, owing to the thickening of the interior of 

 the cardinal region of the valve on each side. 



Surface of each valve ornamented by about sixteen to 

 twenty-four strong, more or less angular radiating plications, 

 of which three or four (rarely five) occupy the mesial sinus, 

 and from four to six the mesial fold ; plications generally sim- 

 ple, but occasionally some of those in the sinus and on the 

 fold, and still more rarely, a few of those on the lateral slopes, 

 bifurcating once ; lines of growth moderately distinct, particu- 

 larly near the free margins of adult shells, where they present 

 a distinctly zigzag appearance in crossing the plications and 

 the furrows between them. Protected portions of the surface 

 of well-preserved specimens also often show, under a strong 

 magnifier, numerous regularly arranged minute granules, prob- 

 ably coincident with the punctures of the shell substance. 

 ( Meek.) 



Horizon and localities. Lower Silurian, Trenton lime- 

 stone : Cape Girardeau. 



Platystrophiaacutilirata (CONRAD). 



Delthyris acutilirata Conrad, 1842: Jour. Acad. Nat. Sci., Phila,, vol. 



VII, p. 260, pi. xiv, fig. 15. 

 Orthis acutilirata Meek, 1873: Geol. Sur. Ohio, Pal., vol. I, p. 119, pi. x, 



figs. 5a-g. 



Like P. lynx but with hinge-line extended, often making 

 the shell twice as wide as long. 



Horizon and localities. Lower Silurian, Hudson shales : 

 Louisiana. 



