104 KANSAS UNIVERSITY QUARTERLY. 
tips of the ears where they are nearly parallel. There are in un- 
weathered, unworn specimens minute pustules arranged in some- 
what radiating order, also linesof growth sometimes visible on front 
border. In exfoliated specimens the fasciculation is less distinct. 
Measurement of average specimen: length, 32 mm.; breadth, 
) 
42 mm.; convexity, 21 mm. 
VARIATIONS. 
The variation in outline is great, as illustrated in figures 2 and 17, 
in Plate VI, and intermediate forms, the former having the hinge 
line extending into mucronate ears, while in the latter the hinge 
line is shorter than the greatest width of the shell. The difference 
in convexity of the shells is shown fairly well in figures 5, 6, 7, 23, 
Plate VI, contrasted with 1, 8, 21, 22. The mesial fold is often 
sharper in the less ventricose forms. 
This species has been considered by some to be identical with 
S. striatus (Martin) Davidson. Schuchert has pointed out a 
difference between the two which is persistent,* and a marked dif- 
ference will be found on comparing with figure g of Plate VI, the 
spires of the two species, as 
shown in the accompanying 
figure (1), which is repro- 
duced by Mr. Prentice from 
Schuchert’s figure in East- 
man’s Translation of Zittel’s 
Hand Book of Paleontology, 
p: 386, (A. dn SS axeeaaes 
the spire is long, loose, and acute at its apex, while in S. 
cameratus the spires are short, compressed, and very obtuse and 
inclose a slightly smaller angle. Even allowing a considerable 
amount of variation to each species there would then be ample 
distinction between the two: furthermore, the compressed ears of 
the attenuate form of .S. cameratus would hardly permit of so long 
a spire. 
The variation of the internal characters of these species is quite 
as remarkable as that of the external portion. The two extreme 
forms will serve to illustrate the great range of these variations. 
In some specimens the muscular scar in the ventral valve is ellipti- 
cal and extends beneath the beak and floor of the foramen, which 
is thick and plate-like; the teeth are not supported by heavy de- 
*Bull. U.S. G. S. 87, p. 884: “The latter species (S. striatus), however, is closely and 
finely reticulated with concentric growth lines, while in 8. camreatus the plications 
are covered with small pustules which are arranged in radiating lines.” 
