414 THE MEDALS OF CREATION. © Cuap. XI. 
tribution is restricted to strata of undoubted fluviatile 
origin, and to those local intercalations of fresh-water and 
land productions in marine deposits, which occur in some 
of the secondary, and in many of the tertiary formations. 
Unio. Ly. p. 28.—The river Mussels, or Unionide, have 
a solid, pearly shell, with two principal and two lateral 
teeth on the hinge ; and their umbones, or bosses, are gene- - 
rally smooth, or longitudinally undulated. Those which 
have no cardinal teeth are arranged under the genus 
Anodon : but it is not necessary for our present purpose, to 
enter into minute conchological distinctions. In number, 
variety, and beauty, the species which inhabit the large 
rivers of North America present a striking contrast with 
the few and homely British fresh-water mussels; nor have 
we, in a fossil state, any shells of this family at all compa- 
rable with those living types.* The earliest fossil Mollusca 
referred to the genus Unio appear in certain layers of clay 
and argillaceous ironstone belonging to the Carboniferous 
system of Derbyshire, Coalbrook Dale, &e. (Ain. Conch. 
vol. i. tab. 33). In the former county, these strata are 
termed mussel-band;+ and some beds constitute a compact 
shell-limestone, which admits of being manufactured into 
vases, &c., and takes a good polish ; the sections of the shells 
in this marble are white, on a dark ground. There is, how- 
ever, considerable doubt whether any of the Carboniferous 
shells really belong to the genus Unio; some geologists 
refer them to Cardinia, a group of sea-shells found espe- 
cially in the Lias. 
The earliest undoubted shells of this genus from the 
British strata, are, I believe, those first discovered by me in 
the strata of Tilgate Forest, (Foss. South D. p. 45, and Foss. 
* See American Journal of Science, vol. xlvii. p. 402, “ Unionide.” 
+ “A solid stratum of ironstone, which extends from Tupton 
Moor to Staveley.” Martin’s Petrificata Derbiensia, pl. xxvii. 
