12 FOSSIL ESTHERLE. 



Dr. S. Fischer assigns a shorter carapace to the female of his E. Hierosolymitana 1 than 

 the one belonging to the male. The squarer carapaces above referred to are rare among 

 the snbovate individuals : some of the latter certainly bear what appear to be ova. 



Of Isaura cycladoides {Estheria giy as) M. Joly has remarked that in its young state 

 it undergoes certain successive changes of form, more or less analogous to the persistent 

 conditions of " Artemia, Branchipus, Apus, Daphnia, Zynceus, Cypris, Zimnadia, and 

 Cyzicus ; " one of these stages being marked by the presence of a horizontal Apus-like 

 carapace, and others being accompanied by varied outlines of the carapace-valves. These 

 observations should make us very careful in the examination of the different forms of 

 carapaces, especially those found associated in the same set of strata, and prepare us for the 

 possible specific identity of dissimilar carapaces. 



Recent Estheria have sometimes so thin a carapace that the valves curl up when dry, 

 like horn-shavings or flakes of quill. In other cases, however, the valves are stout enough 

 to retain their convex oviform or Cycladiform shape when dry. Fossil Estheria also have 

 varied in this respect. We sometimes find in them more variety of ornamentation on one 

 and the same valve than has been observed in single recent specimens. Possibly, however, a 

 closer examination of some of the recent valves might show similar series of modified 

 sculpture on individual specimens. 



A.bout twenty species of Estheria are known to occur in the recent state, and six or 

 seven of the two allied genera Zimnadia and Zimnetis. (See page 5.) These are distinguished 

 respectively either by differences in the form and ornament of the carapace, or by more or 

 less important modifications of the limbs or other organs. Our characterisation of the 

 fossil Estheria must necessarily be independent of the structural differences in the body 

 itself; and it is therefore possible that the limited number of species indicated as fossil, 

 and distributed by one and two through the several great accepted geological formations, 

 might be somewhat enlarged if we set a high value on every recognisable difference in the 

 outline and ornamentation of the valves. I have been careful, on the contrary, to restrict 

 myself as far as possible in setting much value on slight modifications in the fossil Estheria. 



When the umbo of the carapace-valve is near the anterior end, as is most frequently 

 the case, we have a resemblance to some of the subtriangular and subovate Bivalve 

 Molluscs, such as Pisidium, Tellina, &c. ; when the umbo is more nearly central, there 

 is sometimes a resemblance to Avicula or Posidonomya ; and this likeness may be 

 strengthened by the valves of the little Estheria being often wrinkled concentrically, the 

 sharp ridges and neat interspaces being replaced by numerous convex ridges, and nearly 

 all the original structure lost. Still a trace of the peculiar reticulate ornamentation is usually 

 left ; and the superinduced wrinkles are not so evenly convex as is usual in the Avicidida, 

 nor so uniformly marked with parallel concentric striae as is frequently the case with 

 those shells (see PI. I, figs. 31, 32, Iuoceramus Snessii, Oppel). The valves are rarely so 



1 'Abhandl. Akad. Wiss. Munclien,' viii, p. C-19, pi. 20, fig. 8. 



