396 MISSISSIPPIAN BRACHIOPODA 



defined, becoming broader and less well defined toward the front, rounded 

 in the bottom and produced anteriorly in a moderately long, rounded, 

 lingual extension ; beak pointed, a little incurved ; cardinal area high, 

 gently concave with the curvature increasing towards the beak, sloping 

 anteriorly from the hinge-line so that the lower and flatter portion lies 

 at an angle of about 55 degrees to the plane of the valve in the specimen 

 whose dimensions are given above, the triangular, differentiated, verti- 

 cally marked portion of the area lying on each side of the delthyrium is 

 large, the lateral basal angles of this larger triangle lying nearly half way 

 from the basal angles of the delthyrium to the cardinal extremities; 

 delthyrium rather broad, its width between two-thirds and one-half its 

 height ; each lateral slope is marked by from 20 to 24 simple, depressed 

 plications which originate along the cardinal margin and grow suc- 

 cessively smaller towards the cardinal extremities, the last two or three 

 or more being almost obsolete. Internally the length of the dental 

 lamellae in the bottom of the valve but has not been observed, the delthyrial 

 plate is concave transversely with a slight, median, longitudinal ridge, 

 and extends about half way from the apex of the delthyrium to the hinge- 

 line, the free extension of the syrinx is rather broad and is about one- 

 third the length of the fixed delthyrial plate. 



Brachial valve most convex on the mesial line at or near the anterior 

 margin, the surface curving most abruptly to the cardinal margin, some- 

 what impressed towards the cardinal extremities; mesial fold well de- 

 veloped, rounded on top, non-plicate ; beak short, incurved, projecting 

 slightly beyond the cardinal margin ; cardinal area narrow, lying in nearly 

 the plane of the valve; each lateral slope marked by simple, rounded 

 plications, similar in number to those of the opposite valve but more 

 strongly developed. 



The minute surface markings of the shell are not shown on the 

 partially exfoliated specimens examined, but a few small patches are 

 sufficiently well preserved to show that the characteristic "twilled 

 cloth" markings were present. Concentric lines of growth of greater 

 or less strength are commonly distributed irregularly upon both valves. 

 Shell structure minutely punctate. 



Remarks. This species, the type of the genus, has sometimes been 

 considered as a synonym of S. carteri, a species which is imperfectly 

 known, the types of that species being brachial valves which do not ex- 

 hibit the specific or even the generic characters. 



Two groups of spiriferoid shells with greatly elevated cardinal areas 

 are present in the Mississippian faunas, both of which are characterized 

 by the peculiar "twilled cloth" surface markings, the punctate shell 

 structure and usually by the non-plicate fold and sinus. The distinction 

 between the two groups is found in the presence or absence of the syrinx 



