472 REVIEWS 



These volumes have been prepared primarily for the non-specialist, 

 more especially for workers in stratigraphic geology who have not 

 received special training in paleontology. For such workers, as well as 

 for geological students in colleges and universities, and for amateur 

 paleontologists and collectors of fossils, the volumes will prove to be of 

 great value. 



S. W. 



Olenellus and Other Genera of the Mesonacidae. By Charles D. 

 Walcott, Smithsonian Miscellaneous Collections, LV, No. 6. 



In his memoir on the Olenellus fauna, published in the Tenth Annual 

 Report of the United States Geological Survey, in 1891, Walcott recog- 

 nized seven American and three foreign species of Olenellus, included in 

 three subgeneric groups, Olenellus proper, Mesonacis, and Holmia. The 

 present contribution represents the advance of knowledge concerning 

 this highly interesting group of Cambrian trilobites since the appearance 

 of the earlier memoir. Thirty-four species, including two varieties, are 

 now recognized, besides two undetermined ones, thirty-six in all, arranged 

 in ten groups which are given full generic rank, the entire group of forms 

 being elevated to a family under the name Mesonacidae. Twenty-four 

 of these forms are American and twelve foreign, the foreign representa- 

 tives being known only from northwestern Europe. 



With the restriction of the genus Olenellus to include only one group 

 of these species, it comes about that this genus is no longer characteristic 

 of the entire Lower Cambrian, as has commonly been assumed since the 

 publication of the earlier memoir, but occurs only in the uppermost 

 division of the series. In the present paper the Lower Cambrian is 

 divided into four faunal zones, designated, beginning with the oldest, 

 (1) Nevadia zone, (2) Elliptocephala zone, (3) Callavia zone, (4) Olenellus 

 zone, each named from the leading Mesonacid genus present in the fauna. 

 Aside from these four index genera the following are recognized : Meson- 

 acis Walcott, Holmia Matthew, Wanneria n. gen., Paedeumias n. gen., 

 Peachella n. gen., and Olenelloides Peach. 



In their genetic relations the genera discussed are assumed to diverge 

 along two lines from the primitive Nevadia. The one line includes 

 Callavia, Holmia, and Wanneria in order, the last of which is supposed 

 to give origin to Paradoxides of the Middle Cambrian. The second line 

 of descent springing from Nevadia includes Mesonacis, Elliptocephala, 

 Paedeumias, and Olenellus in serial order, the last of these genera giving 

 origin on the one hand to Peachella and on the other hand to Olenelloides. 



