ESTHERIA MEMBRANACEA. 15 



wrinkles a finely granular appearance, which is probably due to the modification of an origi- 

 nally minute reticulate ornament, by the pressure of the sand-granules of the matrix (fig. 5). 

 Some layers of the valve, when exposed by flaking, are quite smooth, amber-coloured, and 

 semi-transparent. The differences between the conditions here described and those 

 observed in the better preserved specimens from Livonia will be pointed out when the 

 latter specimens come to be described presently. 



As it is possible that the difference in shape between the many subquadrate and the 

 few oblong valves may be due to pressure, to a condition of growth, or perhaps to diffe- 

 rence of sex, it would be too hazardous even to distinguish them by name as varietal forms. ^ 

 Indeed, circumstances have so modified the great majority of the valves in the slabs of 

 stone, on the bedding-planes of which they occur, that the whole outline of a valve can very 

 rarely be definitely traced even among a hundred individuals ; for the edges either overlap, 

 are squeezed out of proportion, are broken away, or, lastly, remain buried a little way in 

 the matrix. 



The carapace-valves of this small bivalve Crustacean occur plentifully on some of the 

 surface-planes of the Caithness Flagstones, near Wick, and also in the Orkney and Shet- 

 land Isles, and have been noticed by Hugh Miller and others.^ Their close resemblance 

 to the shells of small bivalve molluscs formerly led to their being taken for the shells of 

 Astarte, Femes, Cyclas, &c. ; but their supposed relation to molluscs having been doubted,^ 

 some specimens from near Thurso, collected by Mr. Peach, were given to me by Mr. 

 Woodward, in 1855 ; and a far larger number, and better preserved, from Kirkwall and 

 Murkle Bay (collected by Mr. Dick), were confided to me for examination by Sir R. 

 Murchison in 1858. 



In their substance, consistence, configuration, and size, these little valves offer direct 

 analogies to the bivalved carapaces of certain recent Phyllopodous Crustaceans {Estheria:) 

 inhabiting the rivers and lagoons of hot countries, and often much resembling the shells of 

 Nucula, Cyrena, &c. 



Great numbers of the valves are spread over large surfaces of the flagstone, sometimes 

 scattered sparsely, sometimes congregated in groups, forming films between the layers of 

 the fissile stone. The valves are usually single ; pairs, with their hinge-lines in juxtaposi- 

 tion, are rare. The specimens which I have are in dark-grey, tough, fine-grained, sandy 



\ In one or two instances I have been almost misled by apparently elongate valves, -which are in truth, 

 two valves pressed accidentally one on another "conformably," but still one extending a little beyond the 

 other. 



2 Miller's 'Old Red Sandstone,' 1st edit., 1841, p. 99, pi. 5, fig. 7 ; 4th edit., 1850, p. 132, pi. 5, 

 fig. 7 ; edit. 1858, p. 116 ; ' Cruise of the Betsey,' 1858, p. 415. Dr. J. G. Malcolmson, ' On the Relations ' 

 of the Old Red Sandstone,' &c. (read in 1839), 'Quart. Journ. Geol. Soc.,' vol. xv. (1859), p. 351 ; Sir R. 

 I. Murchison, Ibid., p. 404, 411, and 413 ; ' Siluria,' 2nd edit., 1859, p. 288 ; C. W. Peach, 'Trans. R. 

 Geol. Soc. Cornwall,' 1855, p. 232. 



3 By Mr. S. P. Woodward, at the Meeting of the British Association at Liverpool, 1854. 



