A local population of E. ornatus, represented by samples of 450 pygidia from shales and 450 from the carbonate interbeds, in a 1-1/2 ft. interval in the upper Lockport Dolomite at Hamilton, Ontario, is described: details include glabellar formula based on 25 cranidia. The first 3 axial tubercles of the pygidium occur at intervals of 4 rings or 3. The numbers of pygidia, with intervals from the first to third tubercles of 8, 7, and 6 rings respectively, closely approximate a binomial in each sample of 450, but the frequencies of different types of tubercle spacing differ significantly between carbonates and shales: tubercles tend to be more closely spaced in the carbonates. The facts are interpreted as evidence of: 1) operation of the Hardy-Weinberg law of genetics: the random distribution of genetically-controlled features indicate that these trilobites were either actually or potentially interbreeding: a corollary is that, although dimorphism has not been recognized, reproduction was necessarily sexual; 2) the conditioning influence of environment: the closer spacing of tubercles in carbonates suggests that they were neither tactile nor luminous but light-controlled, probably light-sensitive organs.

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