Unio. _Ly._ p. 28.--The river Mussels, or _Unionidæ_, have a solid, pearly shell, with two principal and two lateral teeth on the hinge; and their umbones, or bosses, are generally 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 comparable with those living types.[372] 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, &c. (_Min. Conch._ vol. i. tab. 33). In the former county, these strata are termed _mussel-band_;[373] 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, however, 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 especially in the Lias. [372] See American Journal of Science, vol. xlvii. p. 402, "Unionidæ." [373] "A solid stratum of ironstone, which extends from Tupton Moor to Staveley." Martin's Petrificata Derbiensia, pl. xxvii. 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. Tilg. For._ p. 57), and subsequently found in numerous localities of the Wealden.[374] [374] They are figured in Geol. S. E. p. 250; and in Dr. Fitton's Memoir, Geol. Trans, vol. iv. pl. 21. * * * * * [Illustration: Lign. 131. Unio Valdensis. (1/3 _nat._) _Wealden; Isle of Wight._] In 1844 I discovered a large species in the Wealden at Brook Point. I have named it _Unio Valdensis_.[375] I have collected and obtained nearly fifty specimens; they present two varieties, the one contracted and narrow, the other broader and deeper; this difference is probably sexual; the wide and deep shells may be the females; for in the living American Uniones the same characters are observed. Some examples are remarkably well preserved; the ligament remaining in a carbonized state, and the body of the mollusk in the condition of molluskite; even a tint of the original tawny reddish colour of the shell is present. The same species has been found in the Wealden strata, near Tunbridge Wells, by Mr. Barlow, C. E.