4 PALJiIONTOLOGY OF THE UPPER MISSOURI. 



sioiis in the ventral valve, four ; one pair in front of the beak near the middle or in 

 the upper half of the shell, and the others situated one on each side near the car- 

 dinal edge. Shell calcareous. Surface concentrically striated, sometimes with thin 

 extended lamellose edges." 



" In general form these shells somewhat resemble Oholus, but the arrangement of 

 the muscular impressions is different. In Oholus the two central scars have their 

 smaller extremities directed do^vnwards, converging towards each other; but in 

 this genus the arrangement is exactly the reverse." — Billings. 



The six or eight known species of this genus, are small shells, and aU occur near 

 the base of the Silurian System. The group has a Avide geographical range. 



Obolella nana. 



(Plate I, Fig. 3, a, b, c, d.) 



Obolella nana, Meek & Hatden, Proceedings Acad. Nat. Sciences, Philadelphia, October, 1861, 435. — Hatden, Amer. 

 Journ. Sciences, XXXIII, 1861, 2d ser. p. 73.— Dana's Geology, p. 187, Fig. 244 A. 



Shell very small, subcircular or transversely suboval ; moderately convex ; rather thick ; front broadly rounded ; 

 sides more narrowly rounded. Beak of dorsal valve short and obtuse. Ventral valve proportionally a little longer 

 than the other, or about as wide as long, and having a slightly more prominent beak ; without a distinct mesial 

 ridge within ; scar of anterior adductor muscle ? located behind the middle ; impressions of sliding muscles narrow ; 

 cardinal area rather narrow and flattened ; groove for the passage of the peduncle shallow. Surface marked by a 

 few small concentric grooves or impressed strise ; exfoliated specimens showing small regularly disposed radiating 

 striae. ' 



Length of dorsal valve, 0.15 inch; breadth of do., 0.17 inch; convexity, 0.15 inch. Length and breadth of a 

 ventral valve of a smaller specimen each, 0.14 inch. 



This species is so closely allied to Obolella chromaiica of Billings, the type of the 

 genus ("New Species, Lower Sil. Foss.".p. 7), that we were inclined to regard it as 

 specifically identical, imtil we had an opportunity to compare it with specimens of 

 Mr. Billings' species sent to us by him. On comparing it with these, we find it is 

 more convex, and proportionally broader, while its concentric markings are stronger. 

 The substance of its shcU dift'ers in showing radiating striae on the inner lamince 

 of exfoliated specimens. 



Locality and ijositlon, same as last. (No. 1025.) 



Class GASTEROPODA. 



Sue-Class PTEROPODA. 



Order Tliecoisiuata. 



Family CAVOLESTID^. 



Shell non-spiral, symmetrical, more or less elongate-conical, or subglo- 

 bose, curved or straight ; the siibglobose, and a few of the more elongate 

 forms, provided with lateral fissures for the passage of the mantle. 



Animal with two united wing-like fins, but without a foot ; abdomen 

 large ; gills in pairs on the ventral side within the mantle cavity, trans- 

 versely plaited and ciliated. Internal superior organs of generation on 

 the right bide. Lingual teeth 1, 1, 1 (in CavoUna), claAV-shaped. 



