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further hack in past time, we find that our enquiry is greatly 
facilitated, not merely by reason of the reduction of our field of 
observation to a smaller accessible area of fossiliferous deposits, 
but also because the objects themselves become more and more 
reduced, not merely in numbers, but in diversity of forms, 
until at last we arrive at rocks in which the whole class is in- 
eluded in some two or three orders or families. 
Thus in the rocks of Tertiary age Crabs ( Decapoda-Brachy - 
ura) are apparently as abundant as in our recent seas. 
But in the Secondary strata we perceive a visible diminution 
in the short-tailed forms, the earliest of which are, at pre- 
sent, only known in rocks of Oolitic age.* 
Lobsters (. Decapoda-Macrura ), however, are abundant in the 
Oolitic series, and extend back into primary or palaeozoic times, 
the first being found in the Coal-measures. f 
Through all these formations we find representatives of the 
principal living genera, with the exception only of those soft- 
bodied forms which could not be preserved in a fossil state, 
such as the “ Brine-shrimp,” Artemia salina ; Cheirocephalus ; 
and the parasitic Lerneonema , Argulus , Nicothoe , and other 
specialised forms. 
As we scan the record of these old Carboniferous rocks, so rich 
in organic remains, we seem to stand on some lofty beacon-hill, 
whence we can cast our glance upwards and downwards along 
the stream of time. Beneath our feet lie buried the last re- 
presentatives of those aboriginal races now quite extinct, the 
Trilobita and the Eurypterida, whose ancient hosts peopled the 
seas of Devonian and Silurian ages, and reached far away into 
the Cambrian epoch. ' Beside them lie the earliest representa- 
tives known of our modern Decapods, Stomapods, and Isopods, 
then but few and feeble, but now the dominant races of the 
Crustacean class. 
Is this, then, the barrier-reef between the Palaeozoic and 
Neozoic life-periods ? Do we indeed find here the beginning 
of all modern forms of Crustacea, and the ending of all ancient 
ones ? By no means ; nor is there, as we have already 
observed, any period in the whole geological record at which a 
hard and fast line can be drawn dividing the class into recent 
and extinct families. 
Certain groups, such as the Entomostraca, are represented 
throughout . Others, like the Amphipoda, may perhaps extend 
* Oldest known British Crab, Palceinachus longipes, H. Woodward, Forest 
Marble, Malmesbury, Wilts. See “ Quart. Journ. Geol. Soc.,” 1866, VoL 
XXII. p. 493. 
t Anthrapalcemon Grossartii, Salter, Coal Measures, Lanarkshire. See 
u Quart. Journ. Geol. Soc.,” 1861, Yol. XVII. p. 531. 
