SEGMENTS IN TRUNK. 



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 u, the number exhibited by the Phacopid?e 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 n in the 

 thorax and 11-13 m tne pygidium, and Phacops, with n in the thorax and 9-12 in the 

 pygidium, approach very closely to the "average" trilobite, and various species of other 

 genera of the Phacopidje 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 Ddmanites, 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 : Calymcne, Dipleura, Goldius, Proetus, Cyphas- 

 pis, Acidaspis, Phacops, Hausinania, Coronura, Odontochile, Pleur acanthus, Calmonia, Pen- 

 naia, Dalnianites, Probolium, and Cordania. 



The trilobites of the late Paheozoic (Mississippian to Permian) belong, with two pos- 

 sible exceptions, to the Proetidrc, and only three genera, Proetus, 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 

 Proetus to 30 in Anisopyge, and the average, 17.3, is high, as is the average for total num- 

 ber 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 Pcedeumias and Menomonia, the most numerously segmented 

 of all the trilobites. 



The above data may be summarized in the following table : 



This table confirms that made up by Carpenter, and shows even more strikingly the 

 progressive increase in the average number of segments in the trunk throughout the Pake- 

 ozoic. 



While the two trilobites with the greatest number of segments are Cambrian, yet on the 

 average, the last of the trilobites had the more numerously segmented bodies. The multi- 

 segmented trilobites are: 



