INVERTEBRATES. 287 



more oval towards the larger extremity. Septa concave, and 

 separated near the smaller end by spaces between one-fourth 

 and one-fifth their own greater diameter ; the intervals increas- 

 ing somewhat, but not near in proportion to the expansion of 

 the shell, above. Siphuncle subcentral, small where it passes 

 through the septa, but swelling out into globular cavities be- 

 tween. Surface apparently smooth. Length of a specimen 

 imperfect at both extremities, and entirely septate, 4.30 inches; 

 greater diameter at the larger end, apparently not less than 

 3.20 inches; do. at smaller end, 0.80. Our specimen being 

 somewhat distorted, the measurement at the larger end cannot 

 be given definitely. 



This species seems to be related to 0. dilatatum, de Koninck (An. Foss.,p. 

 515, pi. 45, fig. 8 a, 8b, 8 c), but differs in presenting an oval, instead of a 

 circular section, excepting near the smaller extremity ; its siphuncle is also 

 more nearly central than is represented in de Koninck's figure 8a and 8b, pi. 

 45, and differs in its peculiarity of expanding into globular cavities between the 

 septa, as in the group Actinoceras. It seems likewise to differ in its surface 

 markings, being nearly smooth; this, however, may be due, .at least in part, to 

 the exfoliation of the outer layers of the shell. 



Locality and position : McDonough county, Illinois. As the„specimen was 

 found loose, its exact geological position is unknown, but it most probably came 

 from the St. Louis group. 



