1030 THE PALEONTOLOGY OF MINNESOTA. 



LOphiletina sublaxa. 



broadly backward. Of the following two species, 0. sublaxa may be regarded as the 

 type. 



The principal, or perhaps we should say only feature relied on in distinguishing 

 this new generic group from Ophileta on the one side and Euomphalus on the other, 

 is the slit-band. Both of those genera may often have an apertural notch at the 

 terminus of the ridge corresponding to the "band," but there is never anything like 

 a definite band, the lines of growth passing over the ridge without interruption 

 further than is occasioned by changing their direction from obliquely backward to 

 obliquely forward. In Ophiletina, however, as is shown in figures 41 and 47 on plate 

 LXXIV, the ridge is as much of a "slit-band" as in the majority of the Lower 

 Silurian Pleurotomariidce. In Helicotoma, certain species of which are considerably 

 like our Ophiletina angularis, the summit of the corresponding ridge, though never 

 flat nor bearing lunulse, is occasionally margined on each side by a delicate raised 

 line, the result being a " band " that is not greatly different from the kind pertaining 

 to species of Lophospira like L. acuminata (compare fig. 8, plate LXXII1, and fig. 24, 

 plate LXXIV). Another constant and easily recognized difference between Ophiletina 

 and Helicotoma is furnished by the course of the lines of growth across the vertical 

 outer surface of the shell. In the former the lines curve forward in the upper half 

 and just as much backward in the lower, the direction on the whole, therefore, being 

 essentially vertical. In the latter the forward direction continues to the basal 

 angle (compare plate LXXIV, fig. 46 with 15, 16, 22, 33 and 37). 



OPHILETINA SUBLAXA, n. sp., and varieties. 



PLATE LXXIV. FIGS. 4042 and 47. 



Shell small, 13 to 16 mm. In diameter, coiled approximately in one plane, the upper side flat, the 

 lower gently concave; volutions slender, three in number, without the more or less prominent nucleus, 

 hexagonal in transverse section, a little wider than high, the greater part of the last free. Of the six 

 angles the strongest bears the band and lies at the outer edge of the upper side. Within this the space to 

 the suture line is divided into halves, the outer concave, the inner a flat slope, by a second carina. The 

 third is prominent and thin and lies considerably beneath the middle of the outer surface of which the 

 fourth carina forms the base. The fifth and sixth angles are more obtuse, and lie one about the middle 

 of the inner side, the other at the junction of the inner and lower sides. Lines of growth strong, equal, 

 somewhat imbricating, averaging about seven in 2 mm., making a slight retral bend (it is often stronger 

 than in our engraving) in crossing the central angle of the upper face, a very strong and sharp retral bend 

 or loop on the sides and summit of the band-ridge, a slight forward curve on the outer and a retral curve 

 on the lower surface. The band itself is convex and sharply defined, the lunuhc strong. 



The above description is strictly of the northwestern form of the species. Of this we have three 

 specimens, one from each of the three states of Minnesota, Wisconsin and Illinois. We have a fourth 

 specimen (original of fig. 42), found by one of the authors in the lower part of the Stones River group in 

 Tennessee, which differs slightly in several particulars. In the first place the whorls are more slender 

 when viewed from above ; next the bight and width of the volutions are more equal ; then the inner face 

 of the whorls is steeper and rounded rather than angular; finally, the upper side of the shell is concave 

 and the lower flat instead of the reverse. Possibly this Tennessee specimen, which, although a silicifled 



