662 THE PALEONTOLOGY OF MINNESOTA. 
[Eurychilina ventrosa. 
marginal area strongly concave, the outer wall well developed and extending from 
near the post-dorsal angle around the ventral side and about half way up the anterior 
side. In perfect specimens the dorsal angles are prominent. 
The pinched appearance of the central portion of the valves, pitted instead of 
reticulated surface, stronger tubercle, wider sulcus and more abruptly elevated 
marginal area, together with other differences readily distinguish this species from 
E. reticulata, E'. manitobensis and E. longula. 
The original types of the species occurred in a hard limestone, and appeared to 
be without pitting of the surface; but a re-examination proved that the shel! is 
usually exfoliated in specimens obtained by splitting the limestone blocks. The 
Minnesota specimens are mostly preserved in soft shale and in many cases are very 
perfect. 
Formation and locality.—“ Lower Blue limestone” of the Trenton formation, Dixon, Illinois, and 
Mineral Point, Wisconsin; Birdseye or “Glade” limestone, Lebanon, Tennessee; rather abundant in the 
lower third of the Trenton shales (Stictoporella bed) at Minneapolis, St. Paul, Cannon Falls and Oxford 
Mills, Minnesota. 
EURYCHILINA VENTROSA, 2. Sp. 
PLATE XLY, FIGS, 1—3. 
Size.— Without marginal area, length 1.82 mm.; hight 1.08 mm.; thickness 0.8 mm. 
With marginal area, length 2.40 mm.; hight 1.5 mm. 
This species is considerably like H. subradiata but the body of the valve is more 
convex and the outline much more oblique. It is also a little shorter. The marginal 
area has about the same width in the two species but it does not rise so abruptly 
and on the whole is convex in E. ventrosa, while the ends are not produced above 
into sharp angles. The border is peculiar also in front where it is bent so as to form 
an angle of about 45° with the plane of the valves. But the principal peculiarity of 
the border lies in a strong swelling which takes up its entire ventral part. Surface 
of valves with obscure traces of large shallow pits. Tubercle strongly developed. 
The ventral swelling of the marginal area is a peculiar feature, and so far as I 
can see, normal. A similar though weaker and longer swelling occurs in four valves 
found associated with EH. subradiata at Minneapolis. As these specimens however 
are typical of that species in all other respects, they probably represent a variety 
that subsequently changed to the form now called EL. ventrosa. 
Formation and locality.—Upper portion of the Galena shales (base of Fusispira bed) near Cannon 
Falls, Minnesota. 
