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.
FOSSIL PHOLADES.
Pholas. [Lign. 166, fig. 5, 6.]—The common boring bivalve called Pholas, must have attracted the attention of every stroller by the sea-shore, from the numerous perforations in blocks of chalk, and other limestones, occasioned by its operations. Some species burrow in wood, and often commit serious ravages in piles and other submarine works constructed of timber. In the earlier ages of our planet we find evidence of the existence of the same kind of living instruments for the disintegration of floating wood, and the reduction of masses of rock into detritus. But no traces of these shells have been found in strata below the Oolite. One species occurs in the Coral Rag, another in the Kimmeridge Clay; two in the Galt and Greensand; and three or four in the tertiary deposits. In the Crag, blocks of stone are occasionally found with the shells of Pholades occupying the perforations they originally formed and inhabited. But all the specimens I have observed in the Galt, Greensand, and Oolite were xylophagous (wood-eating) species. In the Shanklin Sand, masses of fossil wood, literally honey-combed by the perforations of Pholades, are frequent; but the shells themselves are rare. Mr. Sowerby has figured a beautiful specimen of silicified wood, from Sandgate, with numerous shells of this genus (Pholas priscus. Min. Conch. tab. 581). [Lign. 166, fig. 5], represents a fragment of fossil wood, with three shells in situ; a, a shell seen longitudinally; and below, the rounded anterior extremities of two other shells are exposed.
Masses of wood perforated by Pholades, from which all traces of the shells have disappeared, have given rise to some curious fossil remains, which are often very enigmatical to the young collector. In the Kentish Rag, as for example, in Mr. Bensted's quarry, near Maidstone, large blocks of stone are found, covered with groups of subcylindrical mammillary projections, which are obtuse or rounded at the apex. In some examples the interstices between these bodies are free; in others they are occupied by a reddish brown, friable substance, presenting obscure indications of ligneous structure: and rarely, distinct woody fibres may be observed, the direction of which is transverse, or nearly at right angles, to the mammillated projections. These blocks are, in truth, the stony casts of cavities formed by Pholades, in masses of wood, both the vegetable structure, and the shells, having perished.
In the White Chalk specimens of this kind are occasionally found.
A remarkable fact, relating to some of the specimens from the Iguanodon quarry, remains to be mentioned. Upon breaking off the projections, to ascertain if any traces of the shells of the Pholades remained, we discovered in several, near the apex, a univalve shell, a species of Nerita. [Lign. 166, fig. 6], represents a fragment of stone with two of the casts, which have been broken, and in each, at a, a univalve is imbedded. At b, the ligneous structure of the original wood is visible. The only hypothesis that will account for the appearance of these univalves in their present position, is that of supposing that the Nerites crawled into the cavities made in the mass of timber, after the shells of the Pholades had been removed; and that the wood became imbedded in a sand-bank, and the univalves enclosed in the cavities; the ligneous structure in a great measure perished, and the stony casts of the perforations of the borers, with the imprisoned univalves, remained. The Nerites, as shown in the example figured, do not occupy any particular position in the tubes; one has the apex towards the end of the cavity, and the other lies in a transverse direction.[368]
[368] In a fragment of a perforated column, from Puzzuoli, in my possession, by favour of Sir Woodbine Parish, there were numerous living univalves in the cavities made and previously inhabited by the lithodomi.
TEREDO.