Excerpt from A Monograph of the Fossil Malacostracous Crustacea of Great Britain: I, II. London Clay, Gault, and Greensand
IN examining the zoological characters of the Crustacea which existed during the Eocene period, it is impossible not to be struck by the fact, that notwithstanding the obvious relation in most of them to the members of recent groups, amounting often to an almost typical representation of a family, there is at the same time, probably without exception, such a discrepancy as forbids their association under the same generic formula. In some few cases, indeed, it has been difficult, in consequence of that general destruction of the minute but very important organs about the oral, antennary, and ophthalmic regions, which too often prohibits any very correct appreciation of the relations of the species, to assign to the extinct form its true place amongst its recent allies. But ordinarily there have been, amongst the numerous specimens which I have been so fortunate as to have placed at my disposal, some which have sufficed to indicate their affinities with great probability, if not with absolute precision; and these have led me to adopt the conclusion above stated, which is somewhat at variance with the recognised relation between the Eocene and more recent forms of most other classes of animals, of which representatives are found even in that early member of the tertiary series.
In making the necessary comparisons, and in endeavouring to assign to the different characters and structures their absolute or relative importance, I have found it requisite to investigate with some care the homologies of the different regions and subordinate portions of the carapace, as it happens too often that we are driven to that part, almost exclusively, as the basis of our diagnosis. I will, therefore, make a few remarks on this important element in our generalization, before I enter upon the detailed application of these data to the determination and description of the objects themselves.
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