Mytilus, or Mussel.--There have been found about twenty species of this well-known genus of marine shells in the British strata. They are sparingly distributed through the several formations, from the Silurian to the newer Tertiary. One species (_Mytilus Lyellii_, _Wond._ p. 405, fig. 2) occurs in the Wealden, associated with fresh-water shells. Of the genus termed Modiola, which comprises those mussels that have a rounded anterior termination, nearly forty British species have been discovered; ranging through the fossiliferous strata, from the Silurian to the Crag. A beautiful species (_Modiola elegans._ _Min. Conch._ tab. 9), with the shell generally retaining its pearly coat, is found in the London Clay, and in the limestone of Bognor. An undescribed striated _Modiola_ (which may be named _M. striata_, since the striæ are peculiar), occurs in the Kimmeridge Clay, at Hartwell. Those species of Modiola, which excavate hollows in stones, and inhabit them, are arranged in a genus termed _Lithodomus_. The occurrence of these shells in the remaining erect pillars of the Temple of Jupiter Serapis (_Wond._ p. 106), at Puzzuoli, has afforded important and unequivocal evidence of the physical mutations which that part of Italy has undergone. Two species of Lithodomi have been found, by Mr, Lonsdale, in the Oolite. Pholadomya. _Ly._ p. 272, _fig._ 290.--This genus of shells (established by Mr. James De C. Sowerby in the Min. Conch. 1826), comprises about twenty British fossils, all of which, with but two exceptions, occur in the Lias and Oolite. They are equivalved shells, with the posterior end short, and rounded, and the anterior elongated and gaping. The surface is generally marked with ribs, or alternate elevations and depressions, diverging obliquely from the beaks to the margin. In the clay at Osmington and Radipole, near Weymouth, a large species (_P. æqualis_, _Min. Conch._ tab. 546) is abundant. The Oolite of Brora, in Scotland, contains several species. The only species found in our Chalk, is the beautiful shell (_P. decussatum_), figured _Foss. South D._ tab. XXV. _fig._ 3, and first discovered by me, in 1820, in a bed of Chalk Marl, which at that time was exposed at low-water, at the base of the cliff at Brighton, near the present entrance to the Chain-pier. The same species has since been found at Clayton, Hamsey, Southbourne, and other localities of the Marl. [Sidenote: FOSSIL PHOLADES.]