4 PALAEONTOLOGY OP THE UPPER MISSOURI. 



sions 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 lamcllose edges." 



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

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

 smaller extremities directed downwards, 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 all occur near 

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



Obolella liana. 



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



Obolella nana, MEEK & HAYDEN, Proceedings Acad. Nat. Sciences, Philadelphia, October, 1861, 435. HAYDEN, 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 striae ; 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 chromatica of Billings, the type of the 

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

 specifically identical, until 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 shell differs in showing radiating strias on the inner lamina) 

 of exfoliated specimens. 



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



CLASS GASTEROPODA. 



SUB-CLASS PTEROPODA. 



Order ThccosiiinUi, 



FAMILY CAVOLLNTD^E. 



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

 bose, curved or straight ; the subglobose, 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 side. Lingual teeth 1, 1, 1 (in Cavolina), claw-shaped. 



