RUPERT JONES ON THE FOSSIL ESTHERIiE. 
263 
soft parts of tlie mollusc, now lost, might have borne other evidence.^ 
In nothing are naturalists so much deceived as by the manifold 
mimetic resemblances occurring throughout all kingdoms of nature. 
These are not wanting between different groups of the molluscs them¬ 
selves,! and they are very striking in the case of certain Bivalved 
Crustaceans (forming the subject of this notice), closely resembling 
in general form some of the Molluscous Bivalves. A glance at a 
series of figures of the fossil Estherice reminds us of many well-known 
forms of Lamellibranchiata, such as Posidonomya , Modiola , Myacites , 
Anodon, JJnio , Cyclas, Pisidium , Kellio , Turtonia, JVucula, and others ; 
and indeed some of the species which I have to notice have been re¬ 
ferred by palaeontologists to Posidonomya and other molluscs. It 
has, however, generally been felt that there was a difficulty in the 
exact determination of these little shells; a rigorous examination of 
their form and structure was still wanting, the pocket-lens only, and 
not the microscope, having been brought to bear on them. 
When 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 the old plates and woodcuts; thus jEstheria 
membranacea , when perfectly portrayed, is no longer the triangular 
* A marked instance of palaeontological uncertainty as to the relationships of 
certain bivalves occurs in the case of some of the “ Rhaetic” fossils, thus alluded to 
by Mr. Charles Moore, in the Quart. Jour. Geol. Soc. vol. xvii. p. 502, when de¬ 
scribing them under the generic name “ Axinus, Sowerby:”—“Few shells have 
been subject to greater transposition, or have been placed under so many different 
genera, as those included in the group under notice. Von Credner, in Leonhard 
und Bronn’s Jahrbuch, 1860, p. 307, remarks that one of the Rhaetic species has 
by Roemer been called Venus liassica, but without a figure; by Quenstedt, in Der 
Jura, Opts cloacinus; that Escher notices it, but without naming it, from the Kos- 
sen beds; by Oppel and Suess it is called Schizodus cloacinus; and that it had 
previously been given by Bornemann, but without a figure, as Tceniodon Emaldi 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 Pullas- 
tra 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 Tellinites, Isocardia, Cucullcea, JDonax, Sedgmickia, and Schizodus. It 
is not clear wherein the following shells from Beer-Crowcombe differ from the Axi¬ 
nus of Sowerby ; and his name, having priority, is therefore retained.” 
We must recollect, however, that we have in this case a set of dwarfed shells, 
probably of brackish-water habitat. 
f In a memoir in the Philosophical Transactions for 1835, 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. Paludina and Littorina. 
Siphonaria and Ancylus. Littorina and Phasianella. 
Littorina and Assiminia. Neritina and Nerita. 
Mytilus and Dreissena. Bullia and Terebra. 
Anodon and Iridina. Aporrhais and Rostellaria. 
