Various attempts have been made to show that despite the great variability, trilobites do show a tendency toward a definite number of segments in the body.

Emmrich (1839), noting that those trilobites which had a long thorax usually had a short pygidium, and that the reverse also held true, formulated the law that the number of segments in the trunk was constant (20 + 1) Very numerous exceptions to this law were, however, soon discovered, and while the condition of those with less than twenty-one segments was easily explained, the increasing number of those with more than twenty-one soon brought the idea into total disrepute.

Quenstedt (1837) had considered the number of segments of at least specific importance, and both he and Burmeister (1843) considered that the number of segments in the thorax must be the same for all members of a genus. As first shown by Barrande (1852. p. 191 et seq.), there are very many genera in which there is considerable variation in the number of thoracic segments, and a few examples can be cited in which there is variation within a species, or at least in very closely related species.

Carpenter (1903, p. 333) has tabulated the number of trunk segments of such trilobites as were listed by Zittel in 1887 and finds a steady increase throughout the Palæozoic. His table, which follows, is, however, based upon very few genera.

PeriodNo. of
Genera		Average No. of
body-segments
Cambrian1217.66
Ordovician2318.58
Silurian1619.34
Devonian1020.70
Carboniferous0220.75

Due chiefly to the efforts of Walcott, an increasingly large number of Cambrian genera are now represented by entire specimens, and since these most ancient genera are of greatest importance, a few comments on them may be offered.

The total number of segments can be fairly accurately determined in at least nineteen genera of trilobites from the Lower Cambrian. These include eight genera of the Mesonacidæ (Olenellus was excluded) and Eodiscus, Goniodiscus, Protypus, Bathynotus, Atops, Olenopsis, Crepicephalus, Vanuxemella, Corynexochus, Bathyuriscus, and Poliella. The extremes of range in total segments of the trunk is seen in Eodiscus (9) and Pædeumias (45+), and these same genera show the extremes in the number of thoracic segments, there being 3 in the one and 44+ in the other. Pædeumias probably shows the greatest variation of any one genus of trilobites, various species showing from 19 to 44+ thoracic segments. The average for the nineteen genera is 13.9 segments in the thorax, 3.7 segments in the pygidium, or a total average of 17.6 segments in the trunk. Crepicephalus with 12-14 segments in the thorax and 4-6 in the pygidium, and Protypus, with 13 in the thorax and 4-6 in the pygidium, are the only genera which approach the average. All of the Mesonacidæ, except one, Olenelloides, have far more thoracic and fewer pygidial segments than the average, while the reverse is true of the Eodiscidæ, Vanuxemella, Corynexochus, Bathyuriscus, and Poliella.

The eight genera of the Mesonacidæ, Nevadia, Mesonacis, Elliptocephala, Callavia, Holmia, Wanneria, Pædeumias, and Olenelloides, have an average of 20.25 segments in the thorax and 1.5 in the pygidium, a total of 21.75. If, however, the curious little Olenelloides be omitted, the average for the thorax rises to 22.14 and the total to 23.84. Olenelloides is, in fact, very probably the young of an Olenellus. Specimens are only 4.5 to 11 mm. long, and occur in the same strata with Olenellus (see Beecher 1897 A, p. 191).

Thirty-three genera from the Middle Cambrian afford data as to the number of segments, the Agnostidæ being excluded. The extreme of variation there is smaller than in the Lower Cambrian. The number of thoracic segments varies from 2 in Pagetia to 25 in Acrocephalites, and these same genera show the greatest range in total number of trunk segments, 8 and 29 respectively.