134 MISSISSIPPIAN BRACHIOPODA 



like folds which rarely or never cross the median portion of the valve; 

 the entire surface, when well preserved, marked by exceedingly fine, con- 

 centric lines of growth. Spine bases usually inconspicuous, but rather 

 elongate; slender, sparsely scattered spines are present upon some speci- 

 mens when they occur in a favorable condition of preservation. 



Brachial valve deeply concave with the surface compressed towards the 

 cardinal extremities to form small auriculations, the convex external im- 

 pressions of the valve resemble examples of the pedicle valve except that 

 the umbonal region protrudes beyond the hinge-line either not at all or 

 only slightly. Surface of the valve marked by fine, rounded, more or 

 less flexuous and irregular, radiating costas, entirely similar to those of 

 the opposite valve ; towards the cardinal extremities strong, concentric, 

 wrinkle-like folds, similar to those of the opposite valve, occur, but, un- 

 like those of that valve, at least some of them continue across the median 

 portion of the valve, although much more faintly developed than towards 

 the cardinal extremities ; entire surface, when well preserved, covered by 

 fine, concentric, lines of growth. 



Remarks. This species ranges through the Mississippian formations, 

 although it occurs much more abundantly at certain horizons than others. 

 It is a species belonging to the P. cora group of the genus, and should 

 perhaps not be considered as distinct from that species. On comparison 

 with the figures of the type specimen of P. cora 1 there seems to be no 

 essential features which can be used to distinguish our Mississippian 

 shells, unless it be that the type is less strongly convex transversely. The 

 auriculations are imperfectly preserved in D'Orbigny's type, and do not 

 show in the recent illustration, but in his original figure they are made 

 very large and conspicuous besides being mentioned in his original de- 

 scription, and they may have been destroyed since the original definition 

 of the species was written. These Mississippian shells resemble a common 

 form in the Pennsylvanian faunas which is generally more broadly 

 rounded transversely and has more conspicuous auriculations, agreeing in 

 both these characters with the original P. cord with which they are com- 

 monly identified, and as the Mississippian and Pennsylvanian forms are 

 quite certainly distinct specifically, D'Orbigny's name is retained for the 

 later form. 



Hall described P. ovatus in 1858 from a St. Louis limestone specimen 

 preserved in a somewhat weathered condition. The type specimen does 

 not show the hinge-line, but it does show the surface spines more satis- 

 factorily than most examples. In the more recent collections many speci- 

 mens from the same horizon occur, some in a condition of preservation 

 similar to the type and others showing the entire form of the shell much 

 more satisfactorily, and it is from such examples that the above descrip- 



iPal. Universalis, vol. 1, No. 2 (1903). 



