274 
ORIGINAL ARTICLES. 
Eranchipus , Apus , Dapknia, Lynceus, Cypris> Limnadia, and Oysicus; n 
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, espe¬ 
cially those found associated in the same set of strata, and prepare 
us for the possible specific identity of dissimilar carapaces. 
liecent JEstJierice 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 shape when dry. The fossil Estherice, also, seem to have 
varied in this respect. 
In the fossil Estherice we sometimes find more variety of ornament¬ 
ation on one and the same valve than has usually been observed in 
single recent specimens. Possibly, however, a closer examination of 
the recent carapaces might in nearly all cases show similar series of 
modified sculpture on individual specimens. 
About twenty species of EstJieria are known to occur in the 
recent state, and six or seven of the two allied genera Limnadia and 
Limnetis. (See pp. 267, 268.) 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 characterization of the fossil JEstJierice must necessarily be 
independent of 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 recognizable 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 JEstJierice . 
When the umbo of the carapace-valve is near the anterior end, 
as is most frequently the ease, we have a resemblance to some of the 
subtriangular and subovate Bivalve Molluscs, such as JPisidium , 
Tellina , &c.; when the umbo is more nearly central, there is some¬ 
times a resemblance to an Avicula or a Eosidcnomya; and this like¬ 
ness may be strengthened by the valves of the little JEstheria 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 Aviculidce , nor so uniformly marked with 
parallel concentric striae as is frequently the case with those shells. 
The valves are rarely so quadrate as in the Posidonomya and Inoce - 
ramus ; and neither the wrinkles nor the ridges (whichever may mark 
the valves of the JEstheria) are bent off away from the umbo to follow 
the outline of the produced ears of the shell present in most of the 
Aviculidce , but absent in EstJieria. INTor is there any trace of fur¬ 
rows or teeth on the hinge in EstJieria . 
