VARIATION OF THE SPIRALIA IN SEMINULA ARGENTIA 



(Shepard) Hall. 



BY J. W. BEEDE. 

 With Plate VI. 



Owing to the systematic importance and the rarity of good ma- 

 terial of the brachial framework of the brachiopods, any light on the 

 extent of individual variation of these parts is of considerable impor- 

 tance. In the spring of 1899 the University of Kansas received from 

 Prof. C. N. Gould a set of ten specimens of Seminula argentia 

 (Shepard) Hall that show the position of the spires. These speci- 

 mens were all from the same horizon in the Lower Permian. The 

 University also had another specimen showing these characters, as did 

 a specimen in the writer's collection. Recently the study of these 

 specimens was taken up and some remarkable results developed. 



Both valves of this species are quite convex and not infrequently 

 as broad as long. The older specimens are quite ventricose. How- 

 ever, the shell is subject to a considerable variation in form. Four of 

 the twelve specimens studied w T ere somewhat compressed, but it so 

 happens that three of these approach the normal type very closely, 

 while the fourth does not vary from it greatly. Those showing 

 greatest variation have not been subject to any visible external de- 

 formation. The specimens under discussion are of about the average 

 size and form. 



The normal position of the spire is with its apex pointing to the 

 side, near the line where the valves meet, at, or a little in front of, the 

 middle of the shell, which is also the widest point. In the central 

 part of the shell cavity the edges of the spires nearly meet. In front 

 they flare apart, leaving a large, subcircular opening. For convenience, 

 in this paper, this opening will be spoken of as the frontal aperture 

 of the spiralia. It will also be necessary to orient the specimens so 

 that definite positions may be referred to. For this purpose we will 

 consider the specimens as front toward ( back away from) the observer, 

 with the brachial valve uppermost. 



A specimen from the Topeka limestone, Upper Coal Measures, at 

 Topeka, Kan., shows the spires with the apex of one of them pointing 

 almost directly forward toward the anterior end of the shell, turned 

 through a horizontal angle of about ninety degrees from its normal 

 position, while, as nearly as may be determined from the specimen as 

 cut, the apex of the other one is directed toward the median line of 



(155) 



