INTRODUCTION. 3 



and by this author and other crustaceologists the animals of Esthericc and its allies, the 

 Limnadia and Limnetis, had been already fully made known. Another important result 

 of the application of the microscope to these once obscure organic remains was the deter- 

 mination of the intimate structure of the shell as belonging to crustacean and not to mol- 

 luscan organisms. Whilst the shell of Posidonomya Becheri of the Lower Carboniferous 

 rocks is truly of the molluscan type, 1 that of the so-called Posidonomya minuta and its 

 allies is crustacean. 



One of the fossil Estheria (E. tenella, passing under the name of Posidonomya) wa^ 

 regarded by Agassiz, in 1845, and by Naumann, in 1848, as being related to Cypris ; 

 Dr. Volger, in 1846, suggested of another {E. minuta) that it might be a bivalved Crus- 

 tacean ; and another (E. ovata) was suggestively referred to the Cypris and its allies by 

 Lyell and Morris in 1847. 



In 185G the Rev. W. S. Symonds, P.G.S., favoured me with some well-preserved 

 specimens of the little Triassic Estheria* from Pendock, Worcestershire ; and with the late 

 Prof. J. Quekett's kind assistance I was enabled to see most distinctly the true crustacean 

 character of the tissue of its valves under the microscope. This confirmed an opinion I 

 had long held, and which had been previously advanced by Agassiz and Naumann, 3 by 

 Volger * and by Lyell and Morris, 5 that some of the little fossils known as Posidonomya 

 are not molluscs, but closely allied to the Limnadia, Limnetis, and Estheria, bivalved 

 phyllopodous Crustaceans [Entomostraca) of the present day ; and, indeed, as far as the 

 carapace-valves are concerned, this and the other so-called Posidonomya referred to corre- 

 spond to the Estheria of Riippell and Baird 6 {Isaura, Joly ; Cyzicus, Audouin) . 



Different species of these fossil Esthetics occur in the Devonian rocks (Caithness, Orkney, 

 Livonia, and Russia) ; Carboniferous (Scotland, Northumberland, Lancashire, Derbyshire, 

 Belgium, Prance, Bavaria, and Silesia) ; Permian (Ireland, Saxony, and Russia) ; Triassic 

 (England, Prance, and Germany) ; Rhsetic (Somerset, Gloucestershire, Warwickshire, 

 Worcestershire, and Elgin) ; Oolitic (Skye and Scarborough) ; Purbeck (Dorset) ; and. 

 Wealden (Sussex and Hanover). Others are met with in the coal-fields of Lower Meso- 

 zoic age, in North Carolina and Virginia, and along their north-western extension, forming 



1 Having the late Professor Quekett's authority in deciding the molluscan character of a shell of the 

 Lower Carboniferous Posidonomya from Northumberland, which we examined together under the microscope, 

 I cannot agree with Mr. J. \V. Salter in thinking it probable that the great Posidonomya; of the Carboniferous 

 rocks are crustacean, as suggested in his paper in the 'Annals Nat. Hist.,' 3d ser., 18G0, vol. v, p. 153. 



2 This is the little Triassic shell that has been termed Posidonia, and Posidonomya, minuta ; Posidoniu 

 minuta (Alberti), Goldfuss ; Posidonomya minuta, Bronn, Zieten, Strickland, and others. In Morris's 

 ' Catalogue of British Fossils,' 2nd edit., 1854, it is included in the Crustacea (as Estheria minuta) ; but 

 (apparently from inadvertence) it has not been expunged from the list of molluscs in that work. , 



3 ' Bullet. Soc. Geol. France,' 2nd ser., vol. v, p. 301, and vol. vi, p. 90. 

 * ' Neues Jahrbuch f. Min.' 1846, p. 818. 



5 'Quart. Journ. Geol. Soc.,' vol. iii, p. 2/5, and Lyell' s ' Manual of Geology,' 5th edit., p. 332. 



6 'Proc. Zool. Soc.,' part 17, 1849, p. 87. 



