466 J. Prestwich, Esq., on the 



rive at a deposit, the Penneystone, indicating a long period of repose. This ironstone measure, 

 in its interesting fossils, unfolds to us the existence of numerous marine animals — the great 

 Megalichthys, the Gyracanthus, the Cyathocrinites quinquangularis, together with a considerable 

 number of Testacea* ; but associated with them are a few large Unios, and remnants of vegetables. 

 The slow accumulation, it is probable, totally prevented the preservation of the more delicate 

 parts of plants, and permitted only small and worn portions of stems or branches, which resisted 

 the lengthened exposure, to be preserved from destruction. So tardy was the accumulation of 

 silt, as proved by the fine and homogeneous texture of the stratum, by the existence of shells 

 in all stages of growth, and by their general and uniform dispersion, by the absence of the finer 

 and more delicate parts of plants, and by the presence of parasitical animals attached to the re- 

 mains of fishes and shells, that unless the body were encased in an ironstone nodule its decay 

 appears to have been almost inevitable ; for, as before mentioned, the fossils are, with rare excep- 

 tions, consisting chiefly of the scales and teeth of tlie Megalichthys, found solely in the ironstone 

 nodules, and not in the shale. The numerous Coprolites attest the existence of many fishes. Yet 

 where are their remains ? Here and there only are a few preserved from that destruction to 

 which long exposure most probably consigned the greater bulk of them. 



A considerable interval seems to have elapsed between the accumulation of the Penneystone 

 shale and the deposition of the overlying coarseish sandstone called the flint- coal flint ; for in the 

 north of the coal-field, thick tablets and large blocks of ironstone, containing a peculiarly crystal- 

 line compound of carbonate of iron and carbonate of lime called curlstone, lie on the top of the 

 Penneystone shale ; and in the same situation in the southern districts of Madeley and Broseley 

 occur extensive tabular masses of conglomerate ironstone, varying in thickness from 1 to 12 

 inches, and consisting sometimes entirely of small white quartz angular pebbles, varying in size 

 from a pin's head to a small marble, mixed with grains of white decomposed felspar, and imbed- 

 ded in a matrix of ironstone, with which it occasionally forms thin alternating layers. This con* 

 glomerate also contains small rolled pebbles of ironstone, characterised by the fossils of the Pen- 

 neystone measures. 



We thus have proofs of a transition from a long period of calm and repose to one of some vio- 

 lence, which apparently was continued in a modified form during the deposition of the overlying 

 stratum of sandstone ; for upon leaving the Penneystone, we suddenly lose all traces of animal 



* Mr. T. Bell has recently called my attention to a singular fact connected with recent fresh- 

 water Testacea ; viz. that in old shells the umbones of bivalves and the apices of univalves bear 

 traces of erosion more or less according to the age of the animal. This circumstance appears to 

 hold good in all cases except with light and fragile shells such as the Lymnea. Now it appears 

 to me that this fact may be of considerable value as applied to the discrimination of marine and 

 freshwater shells in a fossil state, and as a test whether several fossil shells supposed to be of 

 freshwater origin, yet found in situations difficult to reconcile with that supposition, are to be 

 considered indicative of the presence of freshwater^ — such as the extensive series classed as Unios. 

 I have examined a considerable suite of shells, apparently belonging to this genus ; but as they 

 are all casts in ironstone, I have not been able to come to any very decided opinion ; neverthe- 

 less, the greater number of specimens exhibited no trace of erosion, but a few, and amongst them 

 several from the Penneystone, decidedly did. 



