900 THE PALEONTOLOGY OF MINNESOTA. 



[Salpingostoma. buelll. 



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 Whitjield. 



PLATE LiXVII, FIGS. 34-37 and ?38. 



Bucania buelli WHITFIELD, 1878, Ann. Kept. Geol. Surv. Wis. for 1877, p. 76. 

 Bucania (Tremanotw?) 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 ram., 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 subaugular, 

 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 1 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 striae 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. 



