40 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



5. volviceramus, Stoliczka. 



Shell extremely inequivalve, the left valve being very gibbous, 

 with its beak greatly elevated and strongly involute ; and the right 

 nearly fiat and subcircular, with its beak very much less elevated, 

 and its surface concentrically striated and more strongly -undulated. — 

 Inocerdmus involutes, Sowerby. 



A more natural arrangement of these groups would be to commence the 

 series with Volviceramus, and then pass to Actinoceramus, followed by Inoce- 

 ramus proper and the succeeding sections ; but it is in accordance with the 

 usual custom to commence with the typical group in such cases. I should 

 also remark, that these sections are not all ecpially distinct from each other, 

 or from the typical forms of the genus; the Mytiloides and Calillus groups 

 showing more tendency to run together and into the typical section of the 

 genus, than they do to approach either Volviceramus or Actinoceramus ; which 

 latter, however, are also quite distinct from Inoceramus proper. 



I have preferred to cite Chenu rather than Brongniart as the author of 

 Calillus, founded on /. Lamarckii as figured by d'Orbigny and Chenu, because 

 it has been stated that the real I. Lamarckii of Brongniart, and the type of 

 ( 'atillus as proposed by him, was founded on fragments of I. Cuvieri, Sowerby. 

 If this is so, of course Catillus of Brongniart would be exactly synoymous 

 with Inoceramus proper of Sowerby. 



Some species of this genus attained a very large size, specimens having 

 been found from two to three feet in diameter. The outer fibrous layer of 

 these large species is generally very thick, particularly near the beaks. 

 Considerable quantities, and sometimes large pieces of it, are met with in the 

 Cretaceous rocks of the Upper Missouri country, particularly in the cal- 

 careous Niobrara division, where none of the inner nacreous layer is to be 

 seen ; thus showing that, owing to some unknown cause, the latter was there 

 more liable to be dissolved. Owing to the fact that the two layers readily 

 separate, the pearly inner layer is, in the argillaceous Fort Pierre group, 

 generally found attached to internal casts of the shell from which the outer 

 layer lias been exfoliated. This inner layer often presents a beautiful pearly 

 duster, and is known to have secreted pearls, as in other genera of the Avicu- 

 I'kIic. The large number of individuals, and the great size of some of the 



