trilobite
Concise Oxford English Dictionary © 2008 Oxford University Press:
trilobite /ˈtrʌɪlə(ʊ)bʌɪt, ˈtrɪ-/
▶noun a fossil marine arthropod of the Palaeozoic era, with a segmented rear part divided longitudinally into three lobes.
– origin C19: from mod. L. Trilobites, from Gk tri- ‘three’ + lobos ‘lobe’.
'trilobite' also found in these Oxford entries:

