SEGMENTS IN TRUNK. 129 



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 great- 

 est 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 Meson- 

 acid;e (Olcnellus was excluded) and Eodiscus, Goniodiscus, Protypus, Bathynotus, Atops, 

 Olenopsis, Crcpicephalus, Vanuxemella, Corynexochus, Bathyuriscus, and Poliella. The ex- 

 tremes of range in total segments of the trunk is seen in Eodiscus (9) and Pcedeumias 

 (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. Pcedeumias probably shows the greatest var- 

 iation of any one genus of trilobites, various species showing from 19 to 44+ thoracic seg- 

 ments. 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. Crcpicephalus 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 Mes- 

 onacidae, except one, Olenelloides, have far more thoracic and fewer pygidial segments than 

 the average, while the reverse is true of the Eodiscida;, Vanuxemella, Corynexochus, Bath- 

 yuriscus, and Poliella. 



The eight genera of the Mesonacidse, Nevadia, Mesonacis, Elliptoccphala, Callavia, Holmia, 

 Wanncria, Pcedeumias, 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 anc l the total to 23.84. Olenelloides is, 

 in fact, very probably the young of an Olcnellus. Specimens are only 4.5 to n 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 seg- 

 ments, the Agnostidas 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. 



The average of thoracic segments for the entire thirty-three genera is 10.5, of pygidial 

 segments 5.9, a total average of 16.4. It will be noted that the thorax shows on the average 

 less and the pygidium more segments than in the Lower Cambrian. If the Agnostidae could be 

 included, this result would doubtless be still more striking. Of the genera considered, 

 Asaphiscus with 7-11 thoracic and 5-8 pygidial segments, Blainia with 9 thoracic and 6-n 

 pygidial, Zacanthoidcs with 9 thoracic and 5 pygidial, and Anomocare with n thoracic and 

 7-8 pygidial segments came nearest to the average. Only a few departed widely from it. 

 The genera tabulated were Acrocephalites, Alokistocarc, Crepicephdus, Karlia, Hamburgia, 

 Corynexochus, Bathyuriscus, Poliella, Agraulos, Dolichometopus, Ogygopsis, Orria, Asaphis- 

 cus, Ncolenus, Burlingia, Blainia, Blountia, Marjumia, Pagetia, Eodiscus, Goniodiscus, Albcrt- 

 ella, Oryctocara, Zacanthoidcs, Anomocare, Anomocarella, Coosia, Conocoryphe, Ctenoce- 

 phalus, Parddoxidcs, Ptychoparia, Sao, and Ellipsocephalus. 



Enough genera of Upper Cambrian trilobites are not known from entire specimens to 

 furnish satisfactory data. Excluding from the list the Proparia recently described by Wal- 

 cott, the average total trunk segments in ten genera is 18, but as most of the genera are 

 Olenidse or olenid-like, not much weight can be attached to these figures. 



For the Cambrian as a whole, the average for sixty-two genera is between 17 and 18 

 trunk segments, which is surprisingly like the result obtained by Carpenter from only twelve 



