REVIEWS 473 



Some question may be raised, perhaps, as to the legitimacy of the 

 assumption of such a phylogenetic origin of Paradoxides. The most 

 diagnostic character of the entire family Mesonacidae is the absence of a 

 facial suture, although well-developed compound eyes are present. 

 Elsewhere among the trilobites, where the free and fixed cheeks have 

 become anchylosed, with the consequent disappearance of the facial 

 suture, as, for instance, in the Devonian genus Phacops, this character 

 has appeared at the termination of a long phylogenetic line in which all 

 the earlier members possess functional facial sutures. The facial suture 

 is so characteristic of every order and every family of trilobites, save 

 the Mesonacidae, that one is forced to the assumption that it was a 

 character of the primitive stock from which all have sprung. It there- 

 fore seems necessary to assume that the ancestors of the Mesonacidae 

 possessed a functional facial suture, and that the absence of this character 

 in this group of genera is indicative of its terminal position in a long 

 phylogenetic line whose pre-Cambrian history is unknown to us. Since 

 such a character when once lost cannot be restored again, it would follow 

 that Paradoxides with its functional facial suture could not have origi- 

 nated from any member of the Mesonacidae. Might it not be assumed 

 that Paradoxides arose from a totally distinct phylogenetic line in a 

 different early Cambrian biologic province, perhaps southern Europe, 

 and later migrated into the North Atlantic province where it occurs 

 in strata generally younger than those bearing the Mesonacid faunas ? 

 Under such an interpretation it would be necessary to grant that some- 

 where Paradoxides may have been contemporaneous with at least a 

 portion of the Mesonacid faunas in North America, and this contem- 

 poraneity may even have extended to the North American shore of the 

 North Atlantic basin. 



The paper adds much to our knowledge of these very ancient faunas 

 of the earth, and the author is to be congratulated upon the success of 

 his most persistent search for these rare fossil forms. Not the least 

 attractive portion of the paper are the twenty-two beautifully executed 

 half-tone plates. 



S. W. 



Elements of Geology. By Eliot Blackwelder and Harlan H. 



Barrows. Pp.475; figs. 485; pis. 16. New York: American 



Book Co., 1911. 

 This is not a manual or reference book, but an elementary textbook 

 intended primarily for use by young students in the high schools, acade- 



