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"Four stages in the development of the trilobite Agnostus nudus. A, Youngest stage with no mesosomatic somites; B and C, stages with two mesosomatic somites between the prosomatic and telsonic carapaces; D, adult condition, still with only two free mesosomatic somites." — The Encyclopedia Britannica, 1910

Agnostus Nudus

"Four stages in the development of the trilobite Agnostus nudus. A, Youngest stage with no mesosomatic…

"Five stages in the development of the trilobite Sao hirsuta. A, Youngest stage. B, Older stage with distinct pygidial carapace. C, Stage with two free mesosomatic somites between the prosomatic and telsonic carapaces. D, Stage with seven free intermediate somites. E, Stage with twelve free somites; the telsonic carapace has not increased in size. a, Lateral eye. g, So-called facial "suture" (not really a suture). p, Telsonic carapace." — The Encyclopedia Britannica, 1910

Sao Hirsuto

"Five stages in the development of the trilobite Sao hirsuta. A, Youngest stage. B, Older stage with…