362 MISSISSIPPIAN BRACHIOPODA 



Surface of each valve marked by 80 or more depressed, rounded, bifur- 

 cating plications, about 20 to 25 of which occupy the fold and sinus. 

 The minute surface markings consist of exceedingly fine radiating striae, 

 about 6 to 10 of which occupy each plication, and by still finer concentric 

 striae which give to the surface of perfectly preserved shells a finely can- 

 cellated ornamentation. Concentric lines of growth are usually dis- 

 tributed over the surface of the shell in an irregular manner, often 

 becoming somewhat crowded anteriorly. 



Internally the muscular impressions in the pedicle valve are strongly 

 impressed, the muscular area is more or less rhombic in outline but varies 

 greatly in proportionate width, in some individuals being twice as 

 wide as in other shells of approximately the same size. The dental 

 lamellse are short and more or less widely divergent, the divergence de- 

 pendent upon the width of the muscular area; between the dental lamella 1 

 towards the beak, at least in old individuals, there is a secretion of cal- 

 careous matter which completely solidifies the shell in that region, this 

 solidification, however, does not extend out to the plane of the surface 

 of the cardinal area, its posterior surface appearing as a transverse area 

 with a median keel or ridge, somewhat deeply depressed in the upper 

 part of the delthyrium, its lower margin being angularly notched. 



Remarks. This species is one of the most common and characteristic 

 fossils of the Burlington limestone, but it usually occurs in a more or 

 less fragmentary condition. Specimens preserving both valves are ex- 

 ceedingly rare, the most common examples being specimens of the pedicle 

 valve which are more or less incomplete about the margin. The nearest 

 relative of 8. grimesi in the faunas of the Mississippi valley is 8. logani 

 of the Keokuk limestone, and in the fragmentary condition in which 

 both species are usually preserved it is often difficult to properly dis- 

 tinguish between them. With complete or fairly complete examples of the 

 two species, it is seen that 8. grimesi is characterized by the shorter hinge- 

 line, rounded cardinal extremities, and proportionally more elongate 

 shell. The elongation of the shell is a variable character in the species 

 as is indicated by the measurements of the two examples given above, 

 the type specimen being an extreme form in which the length is greater 

 than the width, the more usual condition is shown by the measurements 

 of the larger individual having the width greater than the length. From 

 the characters mentioned, especially the proportionate length of the 

 hinge-line, it is not infrequently possible to identify the incomplete ex- 

 amples of the species as they usually occur, since one or more lines of 

 growth can usually be detected. If these lines converge as they approach 

 the cardinal margin the species is clearly a member of the species S. 

 grimesi, but if they diverge it is 8. logani. 



Horizon. Burlington limestone. 



