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.