INVERTEBRATE PALAEONTOLOGY. ;,s:', 



Length of an adult, about 1.86 inches; breadth, 1.30 inches; apical 

 angle convex, divergence 70°. 



Young shells of this fine species must be almost exactly like the last in 

 form, size, and surface-markings ; but at maturity it attains double the size 

 of that shell, and differs in having its last turn rounded instead of angular. 

 It also differs in having its aperture more oval in outline and more produced 

 below. At one time I was inclined to think it might be the adult form oi 

 the last, and the specimens upon which that species was found only young 

 shells. Collections subsequently obtained, however, show that the average 

 size of the V. trochiformis at maturity (among hundreds of specimens) is 

 only about half that of the species under consideration, as may be seen by 

 reference to figure 3, c, of plate 44, which represents a fully-developed adult 

 example of that shell. 



Locality and position. — Ten miles below Fort Union, Dakota, and near 

 the mouth of Powder River, on the Yellowstone, in Montana ; from the Fort 

 Union group (probably Eocene). 



Vivipar us I- >■ i <l j i . var. formosus. 

 Plate 44, figs. 3, a, b. 



It is quite probable that this form may belong to a distinct species from 

 V. Leidyi ; but as I have seen but one specimen of this shell and only two 

 of V. Leidyi, I have no means of ascertaining to what extent they may vary. 

 It differs from the typical form of V. Leidyi in having its spire less elevated, 

 and in being proportionally broader, while its lower whorls are much more 

 flattened, and its suture between not near so deep. In the flattening of its 

 volutions, the more angular character of its body-whorl, as well as in its sur- 

 face-markings, it agrees exactly with our V.. trochiformis; but it attains a 

 much larger size than that shell, and is proportionally broader, while its 

 aperture is narrower in proportion to its height, as well as more produced 

 and more angular below. Its umbilical perforation is also proportionally 

 larger. But the strongest reason for believing it distinct from V. trochi- 

 formis is the fact that that species became proportionally narrower as it 

 increased in size, as may be seen by our figure 2, c, of plate 44. 



Length, 1.52 inches; breadth, 1.20 inches; apical angle a little convex, 

 divergence about 74°. 



Locality and position. — Mouth of the Yellowstone River; in the same 

 position as last. 



