2 FOSSIL ESTHERS. 



different groups of the molluscs themselves, 1 and they are very striking in the case of 

 certain Bivalved Crustaceans (forming the subject of this monograph), closely resembling 

 in general form some of the Molluscous Bivalves. A glance at the accompanying plates 

 illustrating some fossil Bivalved Crustaceans reminds us of many well-known forms of 

 Lamellibranchiata, such as Posidonomi/a, Modiola, Myacitcs, Anodon, Unio, Cyclas, Pisi- 

 dium, KeUia, Turtonia, Nucula, and others ; and indeed some of the species here figured 

 have been referred by palaeontologists to Posidonomija and other molluscs. It has, how- 

 ever, generally been felt that there was a difficulty in the exact determination of these 

 little shells ; still a rigorous examination of their form and structure was wanting, the 

 pocket-lens only, and not the microscope, having been brought to bear on them. 



Being subjected to the microscope, and drawn by means of the camera-lucida, many 

 of these minute shells no longer appear with the outlines given to them by older plates 

 and woodcuts ; thus, Estheria membranacea, when perfectly portrayed, is no longer the 

 triangular " Cyclas " or " Venus " of older figures, but has a semi-orbicular Posidonomya- 

 like form. On the other hand, E. minuta has more of the Pisidium-shape than its old 

 name " Posidonomya " would indicate. The microscope, moreover, exhibits the peculiar 

 superficial ornament so characteristic of the Bivalved Crustacea, and wanting in the Mol- 

 iusca ; but of this ornamentation of the Estheria we had at hand the published illustra- 

 tions and descriptions, by Dr. W. Baird, in the 'Zoological Society's Proceedings,' 1849, &c. ; 



geneva, as those included in the group under notice. Von Credner, in 'Leonhard und Bronn's Jahrbuch,' 

 iSGO, p. 307, remarks that one of the Rhaetic species has by Roerner been called Venus liassica, but without 

 a figure ; by Quenstedt, in ' Der Jura,' Opis cloacinus ; that Escher notices it, but without naming it, from 

 the Kossen beds; by Oppel and Suess it is called Sckizodus cloacinus; and that it had previously been 

 given by Bornemann, but without a figure, as Tceniodon Ewaldi of Dunker. 



" In previous notices of the fossils from this zone, by Mr. Strickland, the Rev. P. B. Brodie, and also 

 by Dr. Wright, reference is made to a shell called Pullastra arenicola, Strickl., which is said to occur very 

 abundantly, but only in casts, and of which no figure has been given ; there is no doubt it belongs to the 

 group under consideration. They have also been included by other English authors under the genera of 

 Teflinites, Isocardia, Cucullaa, Donax, Sedgwickia, and Schizodus. It is not clear wherein the following 

 shells from Beer-Crowcombe differ from the Axinas of Sowerby ; and his name, having priority, is therefore 

 l'etained." 



We must recollect, however, that we have in this case a set of dwarfed shells, probably of brackish- 

 water habitat. 



1 In a memoir in the ' Philosophical Transactions ' for 1S3.), Dr. J. E. Gray treats of "shells having 

 every appearance of belonging to the same natural genus, but inhabited by animals of a very different 

 character" (p. 301) ; and, as examples, he enumerates — 



Pupa and Vertigo. Cytherea and Artemis. 



Vitrina and Nanina. Cyclas and Pisidium. 



Rissoa and Truncatella. Palndina and Littorina. 



Siphonaria and Ancylus. Littorina and Phasianella. 



Littorina and Assiminia. Neritina and Nerita. 



MytiluB and Dreissena. Bullia and Terebra. 



Anodon and Iridina. Aporrhais and Rostellaria. 



