Forty genera of Ordovician trilobites known from entire specimens were tabulated, and it was found that the range in the number of segments in the thorax and pygidium was surprisingly large. _Agnostus_, which was not included in the table, has the fewest, and _Eoharpes_, with 29, the most. While the range in number of segments in the thorax is 2 to 29, the range of the number in the pygidium, 2 to 26, is almost as great. A species of _Dionide_ has 26 in the pygidium, while _Remopleurides_ and _Glaphurus_ have evidence of only 2. The average number of segments in the thorax for the forty genera was 10.15, in the pygidium 8.81, and the average number for the trunk 19. Genera with just 19 segments in the trunk appear to be rare in the Ordovician, a species of _Ampyx_ being the only one I have happened to notice. _Calymene_, _Tretaspis_, _Triarthrus_, _Asaphus_, _Ogygites_, and _Goldius_ come with the range of 18 to 20. _Goldius_, with 10 segments in the thorax and (apparently) 8 in the pygidium, comes nearest to the averages for these two parts of the trunk. _Goldius_, _Amphilichas_, _Bumastus_, _Acidaspis_, _Actinopeltis_, and _Sphærexochus_ are among the genera having 10 segments in the thorax, and there are many genera which have only one or two segments more or less than 10. In most Ordovician genera, thirty-five out of the forty tabulated, the number of segments in the thorax is fixed, and the variation is in any case small. In four of the five genera where it was not fixed, there was a variation of only one segment, and the greatest variation was in _Pliomerops_, where the number is from 15 to 19. This of course indicates that the number of segments in the thorax tends to become fixed in Ordovician time. The variation in the number of segments in the pygidium is, however, considerable. It is difficult in many cases to tell how many segments are actually present in this shield, as it is more or less smooth in a considerable number of genera. Extreme cases of variation within a genus are found in _Encrinurus_, species of which have from 7 to 22 segments in the pygidium, _Cybeloides_ with 10 to 20, and _Dionide_ with 10 to 26. As the number in the thorax became settled, the number in the pygidium became more unstable, so that not even in the Ordovician can the total number of segments in the trunk be said to show any tendency to become fixed. The genera used in this tabulation were: _Eoharpes_, _Cryptolithus_, _Tretaspis_, _Trinucleus_, _Dionide_, _Raphiophorus_, _Ampyx_, _Endymionia_, _Anisonotus_, _Triarthrus_, _Remopleurides_, _Bathyurus_, _Bathyurellus_, _Ogygiocaris_, _Asaphus_, _Ogygites_, _Isotelus_, _Goldius_, _Cyclopyge_, _Amphilichas_, _Odontopleura_, _Acidaspis_, _Glaphurus_, _Encrinurus_, _Cybele_, _Cybeloides_, _Ectenonotus_, _Calymene_, _Ceraurus_, _Pliomera_, _Pliomerops_, _Pterygometopus_, _Chasmops_, _Eccoptochile_, _Actinopeltis_, _Sphærexochus_, _Placoparia_, _Pilekia_, _Selenopeltis_, and _Calocalymene_.