Only sixteen genera of Devonian trilobites were available for tabulation, and it is not always possible to ascertain the exact number of segments in the pygidium, although genera with smooth caudal shields had nearly all disappeared. The number of segments in the thorax had become pretty well fixed by the beginning of the Devonian, _Cyphaspis_ with a range of from 10 to 17 furnishing the only notable exception. The range for the sixteen genera is from 8 to 17, the average 11, the number exhibited by the Phacopidæ which form so large a part of the trilobites of the Devonian. The greater part of the species have large pygidia, and while the range is from 3 to 23, the average is 11.2. _Probolium_, with 11 in the thorax and 11-13 in the pygidium, and _Phacops_, with 11 in the thorax and 9-12 in the pygidium, approach very closely to the "average" trilobite, and various species of other genera of the Phacopidæ have the same number of segments as the norm. In every genus, however, the number of segments in the pygidium is variable, the greatest variation being in _Dalmanites_, with a range of from 9 to 23. The number of segments in the pygidium was therefore not fixed and was on the average higher than in earlier periods. The genera used in the tabulation were: _Calymene_, _Dipleura_, _Goldius_, _Proëtus_, _Cyphaspis_, _Acidaspis_, _Phacops_, _Hausmania_, _Coronura_, _Odontochile_, _Pleuracanthus_, _Calmonia_, _Pennaia_, _Dalmanites_, _Probolium_, and _Cordania_. The trilobites of the late Palæozoic (Mississippian to Permian) belong, with two possible exceptions, to the Pröetidæ, and only three genera, _Proëtus_, _Phillipsia_, and _Griffithides_, appear to be known from all the parts. I am, however, assuming that both _Brachymetopus_ and _Anisopyge_ have 9 segments in the thorax, and so have tabulated five genera. The range in the number of segments in the pygidium is large, from 10 in some species of _Proëtus_ to 30 in _Anisopyge_, and the average, 17.3, is high, as is the average for total number in the trunk, 26.3. _Anisopyge_, a late Permian trilobite described by Girty from Texas, is perhaps the last survivor of the group. It seems to have had 39 segments in the trunk, making it, next to the Cambrian _Pædeumias_ and _Menomonia_, the most numerously segmented of all the trilobites. The above data may be summarized in the following table: Period No. of Av. No. of Av. No. of Av. No. genera segments in segments in of trunk thorax pygidium segments ========================================================== Lower Cambrian 19 13.9 3.7 17.6 Middle Cambrian 33 10.5 5.9 16.4 Entire Cambrian 62 ... ... 17-19 Ordovician 40 10.15 8.81 18.96 Devonian 16 11 11.2 22.2 Late Palæozoic 5 9 17.3 26.3