662 THE PALEONTOLOGY OF MINNESOTA. 



[Eurychillna, 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 shell 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. 



EUEYCHILINA VENTROSA, W. Sp. 

 PLATE XLV, FIGS. 1-3. 



Size.— Without marginal area, length 1.82 mm.; hight 1.08 mm.; thiciiness 0.8 mm. 

 With marginal area, length 2.40 mm.; hight 1.5 mm. 



This species is considerably like E. 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 E. 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 E. ventrosa. 



Formation and locality.— Upiiei portion of the Galena shales (base of Fusispira bed ) near Cannon 

 Falls, Minnesota. 



