Weller — KinderhooJc Faunal Studies. 79 



the former being 36 mm. in extent along the hinge-line, while 

 the largest specimen of the latter is 63 mm. The cardinal 

 area of S. extenuatus is usually perfectly flat or slightly 

 convex, in but one specimen is it slightly concave, while 

 in 8. halli on the other hand, the area is usually concave 

 at least towards the beak, and in some individuals it is 

 strongly concave throughout the entire height. The most 

 important difference between the two species, however, is to be 

 found in the length of the pedicle valve from the beak to the 

 anterior margin. In S. extenuatus this distance is always 

 shorter than the height of the area, while in S. halli it is always 



Fig. 1. Fig. 2. Fig. 3. 



Explanation of Figures. 



Profile views of: — 1. Syringothyris extenuatus from the Chonopectus sand- 

 stone. 2. One of the type specimens of S. halli from Burlington. 3. S. han- 

 nibalensis from the Louisiana limestone at Louisiana, Mo. These outlines 

 show the variation in the angle of forward slope of the cardinal area. 



longer. Perhaps a better manner of expressing this same 

 difference is in the angle of forward slope from the hinge-line 

 of the area of the pedicle valve. In the measurement of this 

 angle the plane of the pedicle valve is assumed to pass through 

 the hinge-line and the lateral margins of the valve, cutting off 

 the anterior extension of the mesial sinus. While these lines 

 do not exactly locate a plane, they do so approximately, and 

 the angle between this plane and the cardinal area may be 

 approximately measured. In nine specimens of S. extenuatus 

 this angle does not vary more than two or three degrees from 

 30°, while in the two type specimens of S. halli perfectly 

 enough preserved to admit of measurements, this angle varies 

 only two or three degrees from 60°. In five specimens of 

 8. hannibalensis Swallow, from the Louisiana limestone at 



