900 THE PALEONTOLOGY OF MINNESOTA. 
[Salpingostoma buelli. 
development was progressive from one to the other in the order named. In the 
first the aperture is never greatly expanded, in the second it flares abruptly at 
maturity, while in the third an expanded mouth is developed at more or less 
frequent intervals. The second type was probably evolved in times preceding the 
Trenton period, but the evolution of the third seems to have been postponed till 
after the close of the Lower Silurian. 
SALPINGOSTOMA BUELLI Whitfield. 
PLATE LXVII, FIGS. 34—37 and 738. 
Bucania buelli WHITFIELD, 1878, Ann. Rept. Geol. Surv. Wis. for 1877, p. 76. 
Bucania (Tremanotus?) buelli WHITFIELD, 1882, Geol. of Wis., vol. iv., p. 224. 
Shell of medium size, consisting of about three and a half volutions. The 
greatest diameter of the shell, just before the development of the expanded aperture, 
varies usually between 35 mm. and 36 mm., while the width of the last volution at 
the same point is about 20.5 mm., and its hight one or two mm. less. Volutions 
appressed, subreniform in section, the hight of the inner ones just a little more than 
half the width; dorsum broadly convex, the sides narrowly rounded or subangular, 
the ventral surface slightly concave where it is in contact with the preceding whorl. 
Umbilicus large, exposing all the inner whorls, with the sutural line deep. Aper- 
ture abruptly expanded, nearly horizontal, slightly raised and gently sinuate in front, 
broadly ovate or subcircular in outline with the hight and width nearly equal, the 
latter usually a little the greater; average hight about 35 mm. Transverse surface 
markings (behind the apertural expansion) consisting of fine sharp lines, three or 
four in | mm., and at intervals increasing with growth from 1 to 2 mm., of a stronger 
wrinkle-like set; both sets sweep backward with very little curvature between the 
sides of the volutions and the slit, joining the latter at an angle of about 60°; the 
transverse striz are crossed by nodular revolving lines, the nodes being arranged in 
such a manner that by changing the direction of the light a diagonal arrangement 
will become more prominent than the longitudinal. Toward and on the apertural 
expansion the revolving lines increase gradually in strength, assuming at the same 
time a radial disposition, while the interpolation of a smaller set produces a distinct 
alternation in size at the margin. Inner surface of aperture perfectly smooth or 
exhibiting a few obscure concentric lines. 
Dorsal slit about 24 mm. long, its edges raised, represented by a narrow rough 
ridge on casts. Behind the slit an ordinary band, appearing raised because bordered 
on each side by a narrow groove. In front of it to the beginning of the expansion 
a similar band; this continues as a gradually diminishing small ridge to the sinus in 
the lip. 
